Compositions and methods for treating cancer

ABSTRACT

Provided are nucleic acid constructs and systems which comprise (i) a first nucleic acid construct encoding a toxin operatively linked to a first promoter and at least one cancer-associated signaling responsive enhancer element; and (ii) a second nucleic acid construct encoding an anti-toxin operatively linked to a second promoter, the second promoter being stronger than the first promoter.Also provided are pharmaceutical compositions comprising same and methods of using same for treating cancer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/574,855, filed Nov. 17, 2017, which is a National Phase of PCT PatentApplication No. PCT/I1L2016/050522 having International filing date ofMay 17, 2016, which claims the benefit of priority of U.S. ProvisionalPatent Application No. 62/162,765, filed May 17, 2015, the contents ofwhich are all incorporated herein by reference in their entirety.

SEQUENCE LISTING STATEMENT

The ASCII filed comprising 25,831 bytes, submitted concurrently with thefiling of this application is incorporated herein by reference. Thesequence listing submitted herewith is identical to the sequence listingforming part of the international application.

FIELD AND BACKGROUND OF THE INVENTION

The present invention, in some embodiments thereof, relates to nucleicacid constructs and nucleic acid construct systems for treating cancerand, more particularly, but not exclusively, to pharmaceuticalcompositions and methods using same for treating cancer.

Colorectal cancer (CRC) is a major health concern in the Western world(American Cancer Society. Colorectal Cancer Facts & Figures. 2014). Theprognosis for metastatic CRC still remains un-satisfactory. Resistanceto chemotherapy is a major obstacle for effective treatment. CRCpatients carrying KRAS (KRAS proto-oncogene, GTPase) mutations are aparticular therapeutic challenge, due to their resistance to anti-EGFRtherapies.

Aberrant activation of the RAS pathway plays an important role in themultistep process of CRC carcinogenesis. Oncogenic RAS stimulates anumber of downstream effectors, that activate several transcriptionfactors that bind to the RAS-responsive DNA element and induce earlyresponse gene expression. The polyoma (Py) virus enhancer consistsflanking overlapping binding sites of the Ets and AP1 transcriptionfactors that are essential for oncogene transcriptional activation(Reddy M A, et al., 1992).

Viral gene therapy is an innovative approach that offers a potentialtreatment for inherited and acquired diseases (Nabel et al., 2004). Itusually involves the generation of replication defective viral particlesthat are capable of stably or transiently introducing a desirabletransgene into cells, resulting in slowing down its' progression(Kootstra N A, et al., 2003; Verma I M, et al., 2005; Young L S, et al.,2006). The most characterized human adenoviruses of serotypes 2 and 5(Ad2 and Ad5, respectively) usually cause mild upper respiratory tractinfections, making them well suited for use in gene therapy.

Adenovirus-based cancer therapy are used for two main strategies: (i)direct tumor cell killing through delivery of replicating oncolyticviruses or non-replicating vectors encoding tumor suppressor genes,suicide genes or anti-angiogenic genes, (ii) destroy primary andmetastatic cancer cells through induction of host antitumor immuneresponses (Kaplan et al., 2005). These approaches offer the potential ofselective tumor cell destruction without damage to normal tissues.Apoptotic genes and tumor suppressor genes are used extensively in thisfield (El-Aneed et al., 2004), alone or in combination withchemotherapy. However, the ability to specifically target tumor cellswith gene transfer is limited, and on the other hand, many normal(non-cancerous) cells are affected as well.

Previous studies have shown that recombinant adenovirus carrying thelethal gene PUMA (p53-upregulated modulator of apoptosis) (generous giftof Bert Vogelstein, Johns Hopkins University, Baltimore) under thecontrol of Ets and AP1-RAS-responsive elements (Py2-SV40-PUMA)suppressed the growth of a variety of tumor cells harboring mutated RAS(Dvory-Sobol H, et al., 2005; Dvory-Sobol H, et al. 2006; Dvory-Sobol H,et al. 2007; Giladi N, et al. 2007; Naumov I, et al. 2012; FitzGerald D,et al., 1991).

The present inventors have also recently shown that the addition ofmultiple RAS-responsive elements (Py4/Py5-SV40-PUMA) further improvedthe growth inhibitory potency of the construct and induced apoptosis inCRC and pancreatic cancer cells in vitro and in vivo (Naumov I, et al.2012; Lisiansky V, et al. 2012). However, escape mechanisms andincreased expression of anti-apoptotic genes can render the cellsresistant as the induced programmed cell death pathway can beinactivated.

MazF is a bacterial ribonuclease known to have specificity for ACAsequences in single-stranded RNA. MazF-induced toxicity is executed byblocking de novo protein synthesis through its endoribonucleaseactivity, termed mRNA interferases (Inouye et al., 2006). In nature,MazF is one of a pair of genes encoding for a stable toxin and anunstable antitoxin organized in a bicistronic operon as a part of aflexible genome (Pandey et al., 2005). The antitoxin interferes with thelethal action of the toxin and neutralizes it s toxicity(Engelberg-Kulka H, et al., 2006; Engelberg-Kulka H, et al., 2005). Thisorganization is a hallmark of toxin-antitoxin (TA) operons. TA systemsare evolutionarily successful entities that are prevalent in lowerorganisms, bacteria and archaea, and they play important roles in adiverse range of cellular activities (Cook et al., 2013). Some TAsystems might behave as selfish elements (found in plasmids), whileothers integrate into host regulatory networks (encoded from thechromosome). The first TA system to be identified was shown to play arole in plasmid maintenance (Thisted T, et al. 1994; 13:1960-8). Once acell loses the plasmid encoding the TA system, the toxin will bereleased from the existing TA complex, given that the antitoxin is moreunstable than the toxin. This results in cell growth inhibition thateventually leads to cell death (Gerdes K, et al., 1986).

Additional background art includes Shapira et al 2013 Cancer Res.73:3303.

SUMMARY OF THE INVENTION

According to an aspect of some embodiments of the present inventionthere is provided a nucleic acid construct comprising:

(i) a first nucleic acid sequence encoding a toxin operatively linked toa first promoter and at least one cancer-associated proliferativesignaling responsive enhancer element;

(ii) a second nucleic acid sequence encoding an anti-toxin operativelylinked to a second promoter, the second promoter being stronger than thefirst promoter.

According to an aspect of some embodiments of the present inventionthere is provided a nucleic acid construct system comprising:

(i) a first nucleic acid construct encoding a toxin operatively linkedto a first promoter and at least one cancer-associated signalingresponsive enhancer element;

(ii) a second nucleic acid construct encoding an anti-toxin operativelylinked to a second promoter, the second promoter being stronger than thefirst promoter.

According to an aspect of some embodiments of the present inventionthere is provided a nucleic acid construct system comprising:

(i) a first nucleic acid construct encoding a toxin operatively linkedto a first promoter and at least one cancer-associated signalingresponsive enhancer element;

(ii) a second nucleic acid construct encoding an anti-toxin operativelylinked to a second promoter;

wherein the first nucleic acid construct is provided at a higherconcentration than the second nucleic acid construct.

According to an aspect of some embodiments of the present inventionthere is provided a method of treating cancer comprising, the methodcomprising introducing into the cancer cells the nucleic acid constructof some embodiments of the invention, or the nucleic acid constructsystem of some embodiments of the invention, wherein the cancer cellsare characterized by hyper activity of the signaling as compared tonon-cancerous cells of the same tissue, thereby treating the cancer.

According to an aspect of some embodiments of the present inventionthere is provided a pharmaceutical composition comprising the nucleicacid construct of some embodiments of the invention or the nucleic acidconstruct system of some embodiments of the invention, and apharmaceutically acceptable carrier or diluents.

According to an aspect of some embodiments of the present inventionthere is provided a composition comprising the nucleic acid construct ofsome embodiments of the invention, or the nucleic acid construct systemof some embodiments of the invention for use in treating cancer, whereincells of the cancer are characterized by hyper activity of the signalingas compared to non-cancerous cells of the same tissue.

According to some embodiments of the invention, the second promotercomprises CMV and the first promoter comprises SV40.

According to some embodiments of the invention, the nucleic acidconstruct is adeno-virus based.

According to some embodiments of the invention, the nucleic acidconstruct is Lenti-virus based.

According to some embodiments of the invention, the cancer-associatedsignaling responsive enhancer element comprises a Ras-responsiveelement.

According to some embodiments of the invention, the Ras-responsiveelement comprises the Ets binding site and/or Ap-1 binding site.

According to some embodiments of the invention, the Ets binding site isset forth by SEQ ID NO:1.

According to some embodiments of the invention, the Ap-1 binding site isset forth by SEQ ID NO:2.

According to some embodiments of the invention, the Ras-responsiveelement comprises the PY2 sequence.

According to some embodiments of the invention, the Ras-responsiveelement comprises at least four repeats of the PY2 sequence.

According to some embodiments of the invention, the PY2 sequence is setforth by SEQ ID NO:3.

According to some embodiments of the invention, the first nucleic acidconstruct and the nucleic acid construct are co-transfected into cellsat a 1 to 0.5 ratio, respectively.

According to some embodiments of the invention, the Ras comprises K-Ras.

According to some embodiments of the invention, the anti-toxin comprisesan RNA silencing agent.

According to some embodiments of the invention, the toxin and theanti-toxin comprise a bacterial-derived toxin anti-toxin system.

According to some embodiments of the invention, the toxin anti-toxinsystem comprise a MazEF system.

According to some embodiments of the invention, the second nucleic acidsequence or the second nucleic acid construct further comprises anon-cancerous associated responsive element for regulating transcriptionof the anti-toxin.

According to some embodiments of the invention, the non-cancerousassociated responsive element comprises the p53 wild type responsiveelement.

According to some embodiments of the invention, the p53 wild typeresponsive element is set forth by SEQ ID NO:14.

According to some embodiments of the invention, the second nucleic acidsequence or the second nucleic acid construct comprises at least 2repeats of the non-cancerous associated responsive element.

According to some embodiments of the invention, the second nucleic acidsequence or the second nucleic acid construct comprises 17 repeats ofthe p53 wild type responsive element.

According to some embodiments of the invention, the first nucleic acidsequence or the first nucleic acid construct further comprises arepressor of a bacterial repressor-operator system, the repressor beingunder a transcriptional regulation of the cancer-associated signalingresponsive enhancer element, and wherein the second nucleic acidsequence or construct comprises an operator of the bacterialrepressor-operator system, such that expression of the repressorinhibits expression of the antitoxin.

According to some embodiments of the invention, the repressor comprisesthe Tetracycline repressor (Tet-R) sequence, and wherein the operatorcomprises the tetracycline operator sequence.

According to some embodiments of the invention, the operator comprisesat least two repeats of the sequence tetracycline operator sequence.

According to some embodiments of the invention, the first nucleic acidsequence or the first nucleic acid construct comprises four repeats ofthe PY2 sequence set forth by SEQ ID NO:2 being upstream and operablylinked to the SV40 minimal promoter region set forth by SEQ ID NO:4, atoxin coding sequence being downstream of and transcriptionallyregulated by the SV40 minimal promoter region, an IRES sequence setforth by SEQ ID NO:7 being downstream and operably linked to the toxincoding sequence, and a Tetracycline repressor set forth by SEQ ID NO: 8being downstream of and operably linked to the IRES sequence.

According to some embodiments of the invention, the second nucleic acidsequence or the second nucleic acid construct comprises a CMV minimalpromoter which comprises two repeats of a tetracycline operator as setforth by SEQ ID NO:9 and an antitoxin coding sequence being downstreamof and operably linked to the CMV minimal promoter.

According to some embodiments of the invention, the second nucleic acidsequence or the second nucleic acid construct comprises at least onecopy of the p53 wild type responsive element set forth by SEQ ID NO:14.

According to some embodiments of the invention, the second nucleic acidsequence or the second nucleic acid construct comprises 17 copies of thep53 wild type responsive element, wherein the 17 copies of the p53 wildtype responsive element are set forth in SEQ ID NO:15.

According to some embodiments of the invention, the cancer comprisescolon cancer.

According to some embodiments of the invention, the cancer compriseslung cancer.

According to some embodiments of the invention, the cancer comprisespancreatic cancer.

According to some embodiments of the invention, the cancer comprisesgastric cancer.

According to some embodiments of the invention, the cancer ischaracterized by a hyperactive RAS GTPase activity.

According to some embodiments of the invention, the RAS is a KRASprotein and wherein the hyperactive KRAS is caused by a G13D mutation inthe KRAS protein set forth by SEQ ID NO:16.

According to some embodiments of the invention, the RAS is a NRASprotein and wherein the hyperactive NRAS is caused by a Q61K mutation inthe NRAS protein set forth by SEQ ID NO:17.

According to some embodiments of the invention, the RAS is a HRASprotein and wherein the hyperactive HRAS is caused by a G12V mutation inthe HRAS protein set forth by SEQ ID NO:18.

According to some embodiments of the invention, the method furthercomprising treating a subject having the cancer by a treatment selectedfrom the group consisting of: chemotherapy, biological therapy,radiotherapy, phototherapy, photodynamic therapy, surgery, nutritionaltherapy, ablative therapy, combined radiotherapy and chemotherapy,brachiotherapy, proton beam therapy, immunotherapy, cellular therapy andphoton beam radiosurgical therapy.

According to some embodiments of the invention, the composition furthercomprising an agent suitable for a treatment selected from the groupconsisting of: chemotherapy, biological therapy, photodynamic therapy,nutritional therapy, brachiotherapy, immunotherapy, and cellulartherapy.

Unless otherwise defined, all technical and/or scientific terms usedherein have the same meaning as commonly understood by one of ordinaryskill in the art to which the invention pertains. Although methods andmaterials similar or equivalent to those described herein can be used inthe practice or testing of embodiments of the invention, exemplarymethods and/or materials are described below. In case of conflict, thepatent specification, including definitions, will control. In addition,the materials, methods, and examples are illustrative only and are notintended to be necessarily limiting.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

Some embodiments of the invention are herein described, by way ofexample only, with reference to the accompanying drawings. With specificreference now to the drawings in detail, it is stressed that theparticulars shown are by way of example and for purposes of illustrativediscussion of embodiments of the invention. In this regard, thedescription taken with the drawings makes apparent to those skilled inthe art how embodiments of the invention may be practiced.

In the drawings:

FIGS. 1A-I depict MazF cassette. FIG. 1A—A schematic illustration of themazF cassette. The RRE-activated MazF cassette was constructed bycloning several elements in the following order (from the N terminus):four repeats of the RAS responsive, Ets (SEQ ID NO:1) and AP-1 (SEQ IDNO:2) binding sites (termed “py2”, SEQ ID NO: 3); SV40 minimal promoter(SEQ ID NO:4); monomeric red fluorescence protein mCherry (SEQ ID NO:5);E. coli MazF ribonuclease (SEQ ID NO:6). FIGS. 1B-F—The toxic effect ofthe mazF-encoding viruses. 1×10⁴ HCT116 cells were seeded in 96-wellplates in complete medium. Median dilutions of the MazF-encoding virusesstarting from 25 MOI were added to the cells on the next day. FIGS.1B-E—images from a light microscopy (FIGS. 1B-C) and from a fluorescentmicroscopy (FIGS. 1D-E) depicting cell survival (all of the microscopeimages are of the same magnification). FIG. 1F—a histogram depictingquantification of cell survival based on an enzymatic MTT assay 72 hoursafter infection. Each bar represents the mean±SD of a set of datadetermined in triplicates. FIGS. 1G-H—1×10⁵ cells were seeded in 12-wellplates in complete medium and infected with the different adenovirusesin 20 MOI for 72 hours. Cell death was measured by FACS after stainingwith Annexin V (FIG. 1G) and RedDot2 (FIG. 1H) dyes. FIG. 1I—a histogramdepicting reporter gene expression (luciferase) under thetranscriptional regulation of the SV40 promoter and the PY4Ras-responsive element in HCT116 cells. Briefly, a nucleic acidconstruct which comprises the PY4 Ras-responsive element upstream of theSV40 promoter being operably linked to a luciferase (reporter gene)coding sequence was used for transfection of HCT116 cells in which Rasis hyperactive. 5×10⁵ HCT116 cells were seeded in 6-well plates. Thenext day, when the cells were about 50% confluent, co-transfection with3 μg (microgram) of PY4-SV40-LUC vector plus 0.3 ng (nanogram) ofpRL-CMV (Promega) was performed using jetPEI™ (Polyplus-transfectionInc, NY, USA) according to the manufacturer's instructions. The Lucactivity was normalized to Renilla Luc activity from a parallelco-transfection.

FIGS. 2A-E depict the MazEF cassette. FIG. 2A—A schematic illustrationof the mazEF cassette with the Ras-responsive elements PY2. Thisconstruct is also referred to as “PY4-TA” or “pAdEasy-Py4-TA”hereinafter. The Ras-responsive element (RRE)-activated MazEF cassettewas constructed by cloning several elements in the following order (fromthe N-terminus to the C-terminus): four repeats of “Py2” [comprising theRas responsive Ets (SEQ ID NO:1) and AP-1 (SEQ ID NO:2) binding sites];SV40 minimal promoter (SEQ ID NO:4); monomeric red fluorescence proteinmCherry coding sequence (SEQ ID NO:5); E. coli MazF ribonuclease (SEQ IDNO:6); internal ribosome entry sites (“IRES”, SEQ ID NO: 7);tetracycline repressor coding sequence (SEQ ID NO:8); CMV minimalpromoter with two copies of the tetracycline operator (SEQ ID NO:9; thesequence of the tetracycline operator is provided in SEQ ID NO:11);green fluorescence protein coding sequence (SEQ ID NO:12); E. coliantitoxin MazE coding sequence (SEQ ID NO:13). FIG. 2B depicts the sameconstruct as in FIG. 1A, yet devoid of the Ras-responsive element Py2.This construct is also referred to as “ΔPY4-TA” hereinafter). FIG. 2C—Aschematic illustration depicting the proposed mode of action of thePY4-TA construct (depicted in FIG. 2A) in cells characterized by ahyperactive Ras. The PY2 elements bind transcription factors in Rashyperactive cells, leading to expression of the mazF toxin and thetetracycline repressor (TetR; shown in pink), which then binds to thetetracycline operator and interferes (e.g., blocks) with the expressionthe maze antitoxin under the control of the CMV minimal promoter. As aresult, the expression of the mazF toxin is higher than the expressionof the mazE antitoxin and the cells are doomed to die. FIG. 2D—Aschematic illustration depicting the proposed mode of action of thePY4-TA construct (depicted in FIG. 2A) in cells characterized by a wildtype Ras (i.e., not hyperactive Ras). Since the cells do not include anhyperactive Ras, the PY2 sites do not activate the transcription of themazF toxin, nor the expression of the TetR which is downstream of themazF toxin coding sequence, and as a result, the activity of MazF isinhibited as compared to the activity of the antitoxin mazE, thusensuring the survival of cells having wild type Ras. FIG. 2E—Westernblot analysis depicting expression of the reporter proteins GFP (whichis translationally fused to the antitoxin mazE coding sequence) andmCherry (which is translationally fused to the toxin mazF codingsequence) in cells characterized by a hyperactive Ras at increasing MOIs(multiple of infection). Thus, when the MOI equals to “1” (a singlevirus particle infecting a single cell) there is only expression of themCherry reporter protein, indicating expression of only the mazF toxin.At increasing MOI to “5” (5 virus particles infecting a single cell),there is also “leakage” of expression of the GFP reporter protein,indicating some degree of expression of the antitoxin maze, yet theexpression of the mCherry is significantly higher.

FIGS. 3A-K—Cells having wild type Ras are protected from thecytotoxicity of MazF due to MazE expression. HT29 cells, with WT (wildtype) RAS, were seeded in 96-well plates. After 24 hours, two-folddilutions of recombinant adenoviruses encoding for MazF or MazEF wereadded for 72 hours. Qualitative examination was performed using lightmicroscopy (left images in each of FIGS. 3A-K) and fluorescencemicroscopy showing expression of mCherry (red fluorescent, middle imagesin each of FIGS. 3A-K) or GFP (green fluorescent, right images in eachof FIGS. 3A-K). Microscopic Magnification×100. FIG. 3A—uninfected cells;FIGS. 3B-C—cells were infected with 1.56 MOI of recombinant adenovirusesharboring the MazEF vector (FIG. 3B) or the MazF vector (FIG. 3C). FIGS.3D-E—cells were infected with 3.12 MOI of recombinant adenovirusesharboring the MazEF vector (FIG. 3D) or the MazF vector (FIG. 3E). FIGS.3F-G—cells were infected with 6.25 MOI of recombinant adenovirusesharboring the MazEF vector (FIG. 3F) or the MazF vector (FIG. 3G). FIGS.3H-I—cells were infected with 12.5 MOI of recombinant adenovirusesharboring the MazEF vector (FIG. 3H) or the MazF vector (FIG. 3I). FIGS.3J-K—cells were infected with 25 MOI of recombinant adenovirusesharboring the MazEF vector (FIG. 3J) or the MazF vector (FIG. 3K). Notethe decrease in expression of mCherry in cells infected with the MazFvector as compared to the expression of mCherry in cells infected withthe MazEF vector. Also note the inhibition of cell growth in cellsinfected with the MazF vector as compared to the cell growth in cellsinfected with the MazEF vector. When visualized under a fluorescencemicroscope, the intoxicated Ad-Py4-SV40-mCherry-MazF-infected cellsshowed very faint red fluorescence, indicating inefficient mCherry-MazFaccumulation. This is due to the ribonuclease activity of MazF thatresults in inhibition of protein synthesis, including its own (Zhang Y,MazF cleaves cellular mRNAs specifically at ACA to block proteinsynthesis in Escherichia coli. Mol Cell 2003; 12:913-23′ Shapira A,Removal of hepatitis C virus-infected cells by a zymogenized bacterialtoxin. PLoS One 2012; 7:e32320). On the other hand, the ribonucleaseactivity of MazF was neutralized by its antidote MazE in cells infectedwith pAdEasy-Py4-TA, as indicated by the presence of both red and greenfluorescence.

FIGS. 4A-G depict eradication of R1 cells by recombinantadenovirus-mediated delivery of the PY4-TA (which includes theMazF-MazE) encoding cassette. FIG. 4A—A schematic illustration depictingthe construction of the mCherry control cassette by cloning themonomeric red fluorescence protein mCherry (SEQ ID NO:5) downstream tothe CMV promoter. 1×10⁴ R1 cells were seeded in 96-well plates. After 24hours, two-fold dilutions of recombinant adenoviruses encoding forPY4-TA (MazF-MazE) or mCherry were added for 72 hours. FIGS.4B-D—Representative pictures of uninfected cells (control, FIG. 4B) andcells that were infected with the PY4-TA cassette with 5 MOI (FIG. 4C)or 10 MOI (FIG. 4D) (magnification of ×100 in all microscopic images).FIG. 4E—A histogram depicting quantification of enzymatic MTT viabilityassay which were performed 72 hours post-infection. Cell survival wasmeasured in R1 cells infected with mCherry (black bars, “R1 mCherry”) orthe PY4-TA (white bars, “R1 TA”) at the infected MOI. The relativefraction of viable cells (relative to uninfected controls) wasdetermined by MTT assay. Each bar represents the mean±SD of a set ofdata determined in triplicates. FIGS. 4F-G—fluorescence microscopicexamination of the infected cells (5 MOI of PY4-TA encoding viruses)showing expression of mCherry (FIG. 4F, red staining) and GFP (FIG. 4G,green staining) of the same microscopic field.

FIGS. 5A-J depict a colony formation assay showing selective eradicationof CRC cells by recombinant adenovirus-mediated delivery of the mazEFencoding cassette. On the day before infection, 5×10⁵ HCT116 (whichinclude an hyperactive Ras) and HT29 (which include wild type Ras) cellswere seeded in 6-well plates and subsequently infected with 25 and 10MOI of the viruses “pAdEasy-Py4-mCherry-MazF-IRES-TetR-CMVmp (with theTet operator)-MazE-IRES-EGFP” (labeled as “PY4-TA”) and“pAdEasy-SV40-mCherry-MazF-IRES-TetR-CMVmp-MazE-IRES-EGFP” (labeled as“APY4-TA”) or left un-infected. After 7 hours, the cells weretrypsinized and seeded at 3-fold dilutions and incubated for 7 days.Surviving colonies were stained with 0.02% crystal violet. Note thesignificant decrease in survival cells treated with the PY4-TA vector atboth MOI concentrations in HCT116 cells as compared to the cells treatedwith the APY4-TA vector. Also, it is noted that the cell growth of theHT29 cells were not affected by treatment with either one of thevectors, thus showing the specificity of the PY4-TA vectors for killingcancer cells such as HCT116 cells with hyperactive Ras and not HT29cells which have wild type KRAS gene.

FIGS. 6A-P—1×10⁴ HCT116 and HT29 cells were seeded in 96-well plates.After 24 hours, two-fold dilutions of recombinant adenoviruses PY4-TAencoding for mazEF were added. FIGS. 6A-D—Microscopic examination of theuninfected cells. FIGS. 6A-B—HCT116 cells; FIGS. 6C-D—HT29 cells. FIGS.6E-J—Microscopic examination of the infected cells (10 MOI) wasperformed 72 hours post-infection. FIGS. 6E-G—HCT116 cells. FIGS.6H-J—HT29 cells. Shown is a light microscopy (FIGS. 6E and 6H), andfluorescent microscopy showing mCherry expression (FIGS. 6F and 61) andGFP expression (FIGS. 6G and 6J). Note the significant decrease inmCherry and GFP expression in HCT116 cells treated with the PY4-TA(mazEF) vector as compared to HT29 cells treated with the same PY4-TA(mazEF) vector. FIG. 6K—A histogram depicting cell survival of HCT116cells (empty bars) or HT29 cells (black bars) treated with the samePY4-TA (mazEF) vector at various MOL Cell survival was determined by anenzymatic MTT viability assay, and the relative fraction of viable cells(relative to uninfected controls) was determined. Each bar representsthe mean±SD of a set of data determined in triplicates. FIGS. 6L-O—HT29cells were infected (10 MOI) in the presence (FIGS. 6N and 6O) orabsence (FIGS. 6L and 6M) of 1 μg/ml tetracycline. The cells wereexamined using a microscope 72 hour post-infection. FIG. 6P—1×10⁴ HCT116cells (having hyperactive Ras) were seeded in 96-well plates. TheAd-Py4-TA encoding viruses were added in several MOIs with (empty bars)or without (filled bars) 1 μg/ml tetracycline 24 hours later. Theenzymatic MTT viability assay was performed after 72 hours. Sizebars=200 μm in all microscopic images shown in FIGS. 6A-J; the imagesshown in FIGS. 6L-O were taken using the same magnification as those inFIGS. 6A-J).

FIGS. 7A-C depict FACS analyses (FIGS. 7A-B) and cell survival (FIG. 7C)of cells infected with the PY4-TA or APY4-TA vectors. 1×10⁵ HCT116 cellswere seeded in 12-well plates in complete medium and infected with thedifferent adenoviruses at 10 MOI for 72 hours. FIGS. 7A-B—FACS analysesshowing cell death after staining with Annexin V (FIG. 7A) or RedDot2(FIG. 7B) dyes. In both analyses (FIGS. 7A-B) the untreated cells areshown in red, the toxin antitoxin-treated cells (infected with thePY4-TA vector) are shown in black, and the cells that were treated withthe RRE deletion cassette (APY4-TA) are shown in green. Note theincrease in cell death in cells treated with the PY4-TA vector ascompared to cells treated with the control ΔPY4-TA vector devoid of thePY4 elements. FIG. 7C—A histogram depicting cell survival followinginfection with the PY4-TA vector or the ΔPY4-TA vector. 1×10⁴ HCT116cells were seeded in 96-well plates. After 24 hours, two-fold dilutionsof recombinant adenoviruses encoding for Py4-TA (empty bars) or ΔPY4-TA(filled bars) were added for 72 hours. The relative fraction of viablecells (relative to uninfected controls) was determined by MTT assay.Each bar represents the mean±SD of a set of data determined intriplicates.

FIGS. 8A-C depict inhibition of tumor growth in mice. Tumors were formedin nude mice by subcutaneous injection of 5×10⁶ HCT116 cells on day 0and were treated twice with intraperitoneal 2×10⁹ PFU/mouse of theindicated viruses. FIG. 8A—A graph depicting fold increase of tumor sizein mice treated with the various viruses. Tumor size was measured at theindicated time points and tumor volumes were calculated. The mean valuesfor each group are shown, and the standard deviation is represented byerror bars for each measurement. The P values for the ΔPy4-TA groupcompared to the PBS group are shown in red and those for the Py4-TAgroup compared to the PBS group are shown in green. Each bar representsthe mean±SD of a set of data determined from six mice. FIGS.8B-C—Imaging was performed on the living organism with the Maestro CRiimaging device (FIG. 8B) and outside the mouse body (FIG. 8C). The redfluorescence dye represents the expression of MazF and the greenfluorescence dye represents the expression of MazE.

FIGS. 9A-C depict the cloning of a mutated KRAS. Mouse cells were stablytransfected with the plasmid described in FIG. 9A, thus expressing themutated KRAS in their genome resulting in a hyperactive ras in thecells. The cells were then transiently transfected with the luciferaseconstruct described in FIG. 9B to test for clones having increasedluciferase expression as a result of the binding of the PY4 ras enhancerelements to the transcription factors downstream of the KRAS hyperactivesignaling pathway in the mouse cells. FIG. 9A—Schematic illustration ofthe KRAS G13D cassette. The mutated kras cassette was constructed bycloning of the KRAS gene [mutation in the coding sequence of amino acidat position 13 in which G (glycine) was replaced by D (aspartic acid)]downstream to the CMV promoter. IRES-GFP sequences were cloneddownstream to the KRAS gene. FIG. 9B—Schematic illustration of aconstruct in which the luciferase coding sequence (SEQ ID NO:20) isunder the regulation of the PY4 ras enhancer elements and the SV40minimal promoter. FIG. 9C—A histogram depicting the fold increase ofluciferase expression in the various clones. It is noted that clone C3exhibits the highest expression of luciferase.

FIG. 10 is a Western blot analysis depicting the expression of GFP intumors infected with AAV6 particles. Tumors were induced (derived fromHT29 cell line) in nude mice. Then a systemic single infecting of thevarious serotypes in several titers was conducted, and after two weeksthe mice were sacrificed, the tumors were removed and the expression ofthe GFP was evaluated by Western blot analysis.

FIGS. 11A-B are schematic illustrations of nucleic acid constructsystems according to some embodiments of the invention. FIG. 11A—Shownin a dual system based on the Ras and p53 responsive elements. The firstnucleic acid construct (Ad-PY4-mCherry-mazF) comprises 4 repeats of thePY2 ras enhancer element, followed by the SV40 minimal promoter,followed by the mCherry coding sequence and the mazF toxin; and thesecond nucleic acid construct (Ad-RGCX17-mazE-GFP) comprises the p53wild type responsive element, followed by the SV40 minimal promoter,followed by the mazE antitoxin coding sequence and the GFP fluorescencereporter gene. FIG. 11B—Shown in a dual system based on the Ras and p53responsive elements. The first nucleic acid construct(Ad-PY4-mCherry-mazF) comprises 4 repeats of the PY2 ras enhancerelement, followed by the SV40 minimal promoter, followed by the mCherrycoding sequence and the mazF toxin; and the second nucleic acidconstruct (Ad-RGCX17-CMV-mazE-GFP) comprises the p53 wild typeresponsive element, followed by the CMV minimal promoter (without theTet operator), followed by the mazE antitoxin coding sequence and theGFP fluorescence reporter gene.

FIG. 12 is a histogram depicting cell viability (determined by an MTTassay) of lung cancer cell lines after co-infection with MazF and MazEin an MOI ratio of 1:0.5, respectively. It should be noted that in thisexperiment both the MazF and MazE constructs were under thetranscriptional regulation of the SV40 minimal promoter. H1975 cells:Ras^(wt)/p53^(mut); H1650 cells: Ras^(wt)/p53^(wt); H2030 cells:Ras^(mut)/p53^(wt); SHP77 cells: Ras^(mut)/p53^(mut).

FIGS. 13A-B are histograms depicting luciferase assay (FIG. 13A) and anMTT assay (FIG. 13B). FIG. 13A—The activity of the PY4 Ras-responsiveelement was tested in H1299 (NRAS oncogene expressing cell line), A549(KRAS oncogene expressing cell line) and T24 (HRAS oncogene expressingcell line). The cells were co-transfected with PY4-luciferase andpRL-CMV (Promega) plasmids. The Luciferase activity was normalized toRenilla Luc activity from a parallel co-transfection. The results showthat PY4 transcription can be activated by 3 Ras variants. FIG.13B—1×10⁴ H1299, H2030, A549, and T24 cells were seeded in 96-wellsplates. On the next day, cells were infected with Ad-PY4-mCherry-mazFcarrying viruses for 72 hours. Cell viability was measured by the MTTassay. The results show that toxin transcription can be activated by 3Ras variants.

FIGS. 14A-C are Western blot analyses of p53 (FIG. 14A), p21 (FIG. 14B)and Tubulin (FIG. 14C) of HCT116 cells after treatment with 5FU. HCT116cells were treated with 50 μM 5FU for 24 hours. Then, total cell lysatewas prepared and subjected to Western blot analysis for p53 (FIG. 14A)and p21 (FIG. 14B) analysis. Tubulin (FIG. 14C) was used as a loadingcontrol and in the analyses of both proteins for normalization.

FIGS. 15A-B depict mRNA levels of various transcripts followingtreatment of HCT116 cells with 5FU. HCT116 cells were treated with 50 μM5FU for 24 hours. Then, RNA was prepared and used as a template for cDNAand semi-quantitative PCR was performed for the following transcripts:p21, Bax, Noxa, Puma, MDM2, 14-3-3o, CD95, Btg2, GADD45, and Survivin.The graph represents the quantification of mRNA levels performed usingthe primers listed in Table 2 in the EXAMPLES section which follows.

FIGS. 16A-H depict crystal violet analysis of cells infected with theviruses according to some embodiments of the invention. 5×10⁵ A549 cells(KRAS nut, p53 wild type; FIGS. 16A-D) and H1650 cells (KRAS wild type,p53 wild type) were seeded in 6-well plates. After 24 hours, the cellswere infected with 10 MOI of the PY4-mazF-mCherry and RGC-mazE-GFPviruses, in a ratio of 1:0.5, respectively (FIGS. 16A and 16C for A549cells; and FIG. 16E for H1650 cells). In parallel those cell lines wereinfected with ΔPY4-mazF-mcherry and RGC-mazE-GFP viruses, in a ratio of1:0.5, respectively (FIGS. 16B and 16D for A549 cells; and FIG. 16F forH1650 cells). The CMV-mCherry vector was used as a control (FIG. 16G).After 7 hours, the cells were trypsinized and seeded in 3-fold dilutionsand incubated for 7 days. Surviving colonies were fixed with 4%formaldehyde in PBS and stained with 0.02% crystal violet.

FIG. 17 is a histogram depicting the efficacy of mazF as evaluated inpancreatic cancer cells. PANC1, Mia Paca2, Colo357 (KRAS mutated cells)and BxPC3 (wild type RAS) cell lines were seeded in 96-well plates.After 24 hours, median dilutions of PY4-mazF-mcherry viruses were added.72 hours later, cell survival was measured the enzymatic MTT assay. Notethat the mazF toxin causes to selective eradication of KRAS mutatedpancreatic cells. % of cell viability was significantly lower in thethree mutated cell lines as compared to the wild type (wt) Kras cells.

DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION

The present invention, in some embodiments thereof, relates to nucleicacid constructs and nucleic acid construct systems for treating cancerand, more particularly, but not exclusively, to pharmaceuticalcompositions and methods using same for treating cancer.

Before explaining at least one embodiment of the invention in detail, itis to be understood that the invention is not necessarily limited in itsapplication to the details set forth in the following description orexemplified by the Examples. The invention is capable of otherembodiments or of being practiced or carried out in various ways.

KRAS mutation is an early event in CRC carcinogenesis. The presentinventors have uncovered that the hyperactive RAS pathway can beexploited, rather than inhibited in order to treat cancer. Thus, thepresent inventors devised a well-regulated toxin anti-toxin (TA) systemderived from E. coli which enables selective control and efficientkilling of tumor cells while sparing normal cells.

As shown in the Examples section which follows, a massive cell death, ina dose-dependent manner, reaching 73% at MOI 10 was seen in mutated KRAScells as compared to 22% in cells with wild type KRAS (FIG. 6K).Increased expression of MazE (the anti-toxin) protected normal cellsfrom any possible internal or external leakage of the system andconfirmed the selectivity, specificity and safety of the targetingsystem. Considerable tumor shrinkage (61%) was demonstrated in vivofollowing MazEF-encoding adenovirus treatment without any side effects(FIGS. 8A-C). These results demonstrate a novel, safe and effective genetherapy for treating cancer which exploits the aberrant hyperactivepathway.

According to an aspect of some embodiments of the invention, there isprovided a nucleic acid construct comprising:

(i) a first nucleic acid sequence encoding a toxin operatively linked toa first promoter and at least one cancer-associated signaling responsiveenhancer element;

(ii) a second nucleic acid sequence encoding an anti-toxin operativelylinked to a second promoter, the second promoter being stronger than thefirst promoter.

According to an aspect of some embodiments of the invention, there isprovided a nucleic acid construct system comprising:

(i) a first nucleic acid construct encoding a toxin operatively linkedto a first promoter and at least one cancer-associated signalingresponsive enhancer element;

(ii) a second nucleic acid construct encoding an anti-toxin operativelylinked to a second promoter, the second promoter being stronger than thefirst promoter.

According to an aspect of some embodiments of the invention, there isprovided a nucleic acid construct system comprising:

(i) a first nucleic acid construct encoding a toxin operatively linkedto a first promoter and at least one cancer-associated signalingresponsive enhancer element;

(ii) a second nucleic acid construct encoding an anti-toxin operativelylinked to a second promoter;

wherein the first nucleic acid construct is provided at a higherconcentration than the second nucleic acid construct.

As used herein the term “system” refers to at least two distinct nucleicacid construct molecules.

A coding nucleic acid sequence is “operably linked” or “operativelylinked” (which is interchangeably used herein) to a regulatory sequence(e.g., promoter) if the regulatory sequence has a transcriptionalregulatory effect on the coding sequence linked thereto.

As used herein, the term “promoter” refers to a region of DNA which liesupstream of the transcriptional initiation site of a gene to which RNApolymerase binds to initiate transcription of RNA. The promoter controlswhere (e.g., in which cells) and/or when (e.g., at which stage orcondition in the lifetime of an organism) the gene is expressed.

The promoter can direct transcription of the polynucleotide sequenceoperably linked thereto in a constitutive or inducible manner.

According to some embodiments of the invention, the promoter isheterologous to the coding sequence operably linked thereto.

As used herein the phrase “heterologous promoter” refers to a promoterfrom a different gene locus as of the coding sequence operably linkedthereto.

According to some embodiments of the invention, the promoter comprisesthe minimal promoter sequence required for transcription of the codingsequence operably linked to the promoter.

Assays for determining the minimal promoter sequence are known in theart, and described, for example, in Byrne B J., et al., 1983[“Definition of the simian virus 40 early promoter region anddemonstration of a host range bias in the enhancement effect of thesimian virus 40 72-base-pair repeat. Proc Natl Acad Sci USA. 80(3):721-725], which is fully incorporated herein by reference in itsentirety.

For example, the minimal SV40 promoter sequence is set forth by SEQ IDNO:40; and the minimal CMV promoter sequence is set forth by SEQ ID NO:19.

According to some embodiments of the invention, the second promoter isstronger than the first promoter.

It should be noted that promoter activity can be detected and evaluatedby various methods, such as by operably linking thereto a codingsequence of a reporter protein which can be detected and quantified incells transfected with the construct. Examples of such assays include,but are not limited to using the luciferase coding sequence (e.g., SEQID NO: 20) under the control of the promoter to be tested, and measuringluciferase activity in the transfected cells (e.g., as described inFIGS. 9B and 13A). Thus, for example, selection of suitable first andsecond promoters can be performed by comparing the transcription abilityof these promoters under identical assay conditions, using for example,the same reporter coding sequence.

According to some embodiments of the invention, the second promoterexhibits at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%,150%, 200%, 300%, 400%, 500%, e.g., at least 1000%, higher transcriptionability to transcribe a coding sequence operably linked thereto.

As used herein the phrase “cancer-associated signaling responsiveenhancer element” refers to a nucleic acid sequence which serves as anenhancer of transcription by the binding of a specific transcriptionfactor thereto, wherein the specific transcription factor is expressedin cancer cells.

Typically the cancer-associated signaling responsive enhancer element isa cancer-associated proliferative signaling responsive enhancer element.

Known proliferation signaling of cancer cells include the pathways usedby several oncogenes. For example, in the RAS pathway the proliferationsignaling results in binding of Ets and/or the AP-1 transcriptionfactors to specific Ets and/or Ap-1 binding sites, respectively (whichform the PY2 enhancer element). Similarly, in the WNT signaling pathwaythe proliferation signaling results in binding of the TCF/LEFtranscription factors [e.g., TCF7 (TCF-1), TCF7L1 (TCF-3), TCF7L2(TCF-4) and/or LEF1] to their binding sites (enhancers). Another exampleincludes the MAPK pathway in which the transcription factor MYC (c-myc)can bind to specific binding sites as described elsewhere [Karen I.Zeller, et al., 2006. Global mapping of c-Myc binding sites and targetgene networks in human B cells. Proc Natl Acad Sci USA; 103(47):17834-17839, which is fully incorporated herein by reference].

Similarly, known proliferation signaling of cancer cells include thepathways used by tumor suppressor genes as in the case of retinoblastoma(Rb) tumor suppressor protein. The Rb tumor suppressor protein binds tothe E2F1 transcription factor and prevents the interaction between E2F1and the cell's transcription machinery. In the absence of Rb [e.g., whencyclin-dependent kinases (CDK) and cyclins phosphorylate Rb to pRb],E2F1 (along with its binding partner DP1) mediates the trans-activationof E2F1 target genes that facilitate the G1/S transition and S-phase.E2F targets genes encode proteins involved in DNA replication (forexample DNA polymerase, thymidine kinase, dihydrofolate reductase andcdc6), and chromosomal replication (replication origin-binding proteinHsOrc1 and MCM5). When cells are not proliferating, E2F DNA bindingsites [(e.g., TTTCCCGC (SEQ ID NO:53)] contribute to transcriptionalrepression.

Thus, according to some embodiments of the invention thecancer-associated signaling responsive enhancer element comprises theE2F DNA binding site(s). In this case, when the nucleic acid constructor system thereof is introduced to cancer cells with a mutation in theRb tumor suppressor (which prevents binding of Rb to E2F1) the E2F1transcription factor in the cancer cells can bind to the E2F DNA bindingsites (e.g., SEQ ID NO:53), resulting in activation of the promoteroperably linked to the toxin coding sequence, and with high expressionof the toxin within the cells.

According some embodiments of the invention, the cancer-associatedproliferative signaling responsive enhancer element comprises aRas-responsive element.

According some embodiments of the invention, the Ras comprises K-Ras.

According some embodiments of the invention, the Ras-responsive elementcomprises the Ets binding site and/or the Ap-1 binding site.

The Ets binding site comprises an ETS domain, which is a wingedhelix-turn-helix structure that binds to DNA sites with a centralGGA(A/T) DNA sequence. The ETS family includes 12 subfamilies asdescribed in Table 1 below.

TABLE 1 Subfamily Mammalian family members ELF ELF1, ELF2 (NERF), ELF4(MEF) ELG GABPα ERG ERG, FLI1, FEV ERF ERF (PE2), ETV3 (PE1) ESE ELF3(ESE1/ESX), ELF5 (ESE2), ESE3 (EHF) ETS ETS1, ETS2 PDEF SPDEF (PDEF/PSE)PEA3 ETV4 (PEA3/E1AF), ETV5 (ERM), ETV1 (ER81) ER71 ETV2 (ER71) SPI SPI1(PU.1), SPIB, SPIC TCF ELK1, ELK4 (SAP1), ELK3 (NET/SAP2) TEL ETV6(TEL), ETV7 (TEL2)

According some embodiments of the invention, the Ets binding site is setforth by SEQ ID NO:1.

According some embodiments of the invention, the AP-1 binding site isset forth by SEQ ID NO:2.

According some embodiments of the invention, the Ras-responsive elementcomprises the PY2 sequence. The PY2 sequence comprises the Ets and Ap-1binding site and is set forth by SEQ ID NO: 3.

According some embodiments of the invention, the Ras-responsive elementcomprises at least two repeats of the PY2 sequence, e.g., at least threerepeats of the PY2 sequence, e.g., at least four repeats of the PY2sequence, e.g., at least five repeats of the PY2 sequence or more.

According some embodiments of the invention, the Ras-responsive elementcomprises four repeats of the PY2 sequence (the four repeats of the PY2sequence are also referred to as “PY4” herein).

As used herein the term “toxin” refers to a polypeptide capable ofkilling cells.

As used herein the term “anti-toxin” (or “antitoxin”) refers to an RNAor a polypeptide capable of neutralizing the effect of the toxin in thecells where the toxin is present and/or active.

There are three known types of toxin-antitoxin systems. Type Itoxin-antitoxin systems rely on the base-pairing of complementaryantitoxin RNA with the toxin's mRNA. Translation of the mRNA is theninhibited either by degradation via RNase III or by occluding theShine-Dalgarno sequence or ribosome binding site. Often the toxin andantitoxin are encoded on opposite strands of DNA. Known examples of typeI toxin and anti-toxin pairs which can be used in the construct orsystem(s) of constructs according to some embodiments of the inventioninclude, but are not limited to Hok-Sok [e.g., the Hok protein (SEQ IDNO:47; encoded by SEQ ID NO: 48) and the Sok coding sequence (SEQ ID NO:49) encoding the SOK protein (SEQ ID NO:50)], fst-RNAII, TisB-IstR,LdrD-RdlD, FlmA-FlmB, Ibs-Sib, TxpA/BrnT-RatA, SymE-SymR, andXCV2162-ptaRNA1 [reviewed in Sabine Brantl, and Natalie Jahn. sRNAs inbacterial type I and type III toxin-antitoxin systems. FEMS MicrobiologyReviews. First published online: 25 Mar. 2015. doi:10.1093/femsre/fuv003; Vogel J, et al., 2004. “The small RNA IstRinhibits synthesis of an SOS-induced toxic peptide”. Curr. Biol. 14(24): 2271-6; Greenfield T J, et al., 2000. “The antisense RNA of thepar locus of pADI regulates the expression of a 33-amino-acid toxicpeptide by an unusual mechanism”. Mol. Microbiol. 37 (3): 652-60; KawanoM, et al., 2002. “Molecular characterization of long direct repeat (LDR)sequences expressing a stable mRNA encoding for a 35-amino-acidcell-killing peptide and a cis-encoded small antisense RNA inEscherichia coli”. Mol. Microbiol. 45 (2): 333-49; Loh S M, et al.,1988. “Nucleotide sequence and transcriptional analysis of a thirdfunction (Flm) involved in F-plasmid maintenance”. Gene 66 (2): 259-68;Fozo E M, et al., 2008. “Repression of small toxic protein synthesis bythe Sib and OhsC small RNAs”. Mol. Microbiol. 70 (5): 1076-93; SilvaggiJ M, et al. 2005. “Small Untranslated RNA Antitoxin in Bacillussubtilis”. J. Bacteriol. 187 (19): 6641-50; Gerdes K, et al. 2007. “RNAantitoxins”. Curr. Opin. Microbiol. 10 (2): 117-24; Findeiss S, et al.2010. “A novel family of plasmid-transferred anti-sense ncRNAs”. RNABiol. 7 (2): 120-4; each of which is fully incorporated herein byreference in its entirety].

In Type II toxin-antitoxin systems a labile protein antitoxin tightlybinds and inhibits the activity of a stable toxin. Known examples oftype II toxin and anti-toxin pairs which can be used in the construct orsystem(s) of constructs according to some embodiments of the inventioninclude, but are not limited to, CcdB-CcdA, ParE-ParD, MazF-MazE,yafO-yafN, HicA-HicB, Kid-Kis, and Zeta-Epsilon [reviewed in Bahassi EM, et al., 1999. “Interactions of CcdB with DNA gyrase. Inactivation ofGyra, poisoning of the gyrase-DNA complex, and the antidote action ofCcdA.”. J Biol Chem 274 (16): 10936-44; Jensen R B, Gerdes K, 1995.“Programmed cell death in bacteria: proteic plasmid stabilizationsystems.”. Mol Microbiol 17 (2): 205-10; Singletary L A, et al. 2009.“An SOS-Regulated Type 2 Toxin-Antitoxin System”. J. Bacteriol. 191(24): 7456-65. doi:10.1128/JB.00963-09. PMC 2786605. PMID 19837801;Jorgensen M G et al. 2009. “HicA of Escherichia coli defines a novelfamily of translation-independent mRNA interferases in bacteria andarchaea.”. Journal of Bacteriology 191 (4): 1191-1199; Diago-Navarro E,et al., 2010. “parD toxin-antitoxin system of plasmid R1-basiccontributions, biotechnological applications and relationships withclosely-related toxin-antitoxin systems”. FEBS J. 277 (15): 3097-117;Mutschler H and Meinhart A. 2011. “/(systems: their role in resistance,virulence, and their potential for antibiotic development.”. Journal ofMolecular Medicine 89 (2): 1183-1194; each of which is fullyincorporated herein by reference in its entirety].

Type III toxin-antitoxin systems rely on direct interaction between atoxic protein and an RNA antitoxin. The toxic effects of the protein areneutralized by the RNA gene. A non-limiting example of type III toxinand anti-toxin pairs which can be used in the construct or system(s) ofconstructs according to some embodiments of the invention is the ToxINsystem from the bacterial plant pathogen Erwinia carotovora. The toxicToxN protein is approximately 170 amino acids long and has been shown tobe toxic to E. coli. The toxic activity of ToxN is inhibited by ToxIRNA, an RNA with 5.5 direct repeats of a 36 nucleotide motif(AGGTGATTTGCTACCTTTAAGTGCAGCTAGAAATTC, SEQ ID NO:24).

The sequences of the various toxin and anti-toxin agents (e.g., RNA orproteins) are known in the art and can be obtained from various sourcesincluding the “National Center for Biotechnology Information” data base[www(dot)ncbi(dot)nlm(dot)nih(dot)gov/].

According some embodiments of the invention, the anti-toxin is apolypeptide capable of neutralizing the effect of the toxin in the cellswhere the toxin is present and/or active.

According some embodiments of the invention, the anti-toxin comprises anRNA silencing agent.

According some embodiments of the invention, the toxin and theanti-toxin comprise a bacterial-derived toxin anti-toxin system.

MazF is a bacterial ribonuclease (e.g., SEQ ID NOs: 6 and 51), which isspecific for ACA sequences in single-stranded RNA. MazF-induced toxicityis executed by blocking de novo protein synthesis through itsendoribonuclease activity (mRNA interferases; Inouye et al., 2006). TheMazE antitoxin (e.g., SEQ ID NOs: 13 and 52) interferes with the lethalaction of the MazF toxin and neutralizes it s toxicity.

According some embodiments of the invention, the toxin anti-toxin systemcomprise a MazEF system.

According some embodiments of the invention, the MazF toxin comprisesthe coding sequence set forth by SEQ ID NO:6.

According some embodiments of the invention, the MazF toxin protein isset forth by SEQ ID NO: 51.

According some embodiments of the invention, the MazE anti-toxincomprises the coding sequence set forth by SEQ ID NO:13.

According some embodiments of the invention, the MazE anti-toxincomprises the amino acid sequence set forth by SEQ ID NO: 52.

According some embodiments of the invention, the second promotercomprises CMV and the first promoter comprises SV40.

According some embodiments of the invention, the second nucleic acidsequence or the second nucleic acid construct further comprises anon-cancerous associated responsive element for regulating transcriptionof the anti-toxin.

Thus, by inclusion of the additional responsive element for regulatingthe transcription of the anti-toxin the construct or the constructsystem ensures that no-cell killing of “normal” (non-cancerous) cells.

For example, as shown in FIGS. 11A and 11B the p53 wild type responsiveelement (such as provided in SEQ ID NO:14) can be added upstream of thepromoter which drives the transcription of the antitoxin codingsequence.

According some embodiments of the invention, the non-cancerousassociated responsive element comprises the p53 wild type responsiveelement.

According some embodiments of the invention, the p53 wild typeresponsive element is set forth by SEQ ID NO:14.

According some embodiments of the invention, the second nucleic acidsequence or the second nucleic acid construct comprises at least onecopy of the p53 wild type responsive element set forth by SEQ ID NO:14.

According some embodiments of the invention, the second nucleic acidsequence or the second nucleic acid construct comprises at least 2repeats of the non-cancerous associated responsive element, e.g., atleast 3, at least 4, at least 5, at least 6, at least 7, at least 8, atleast 9, at least 10, at least 11, at least 12, at least 13, at least14, at least 15, at least 16, at least 17, at least 18, at least 19, atleast 20 repeats of the non-cancerous associated responsive element.

According some embodiments of the invention, the second nucleic acidsequence or the second nucleic acid construct comprises 17 repeats ofthe p53 wild type responsive element.

According some embodiments of the invention, the second nucleic acidsequence or the second nucleic acid construct comprises the RCGX17sequence (17 repeats of the p53 wild type responsive element) as setforth by SEQ ID NO: 15.

For example, as shown in FIG. 12, cancerous cells having a mutation inboth ras and p53 (e.g., SHP77 cells which are Ras^(mut)/p53^(mut)) weremore sensitive to cell killing than cells which exhibit wild typesequences of both ras and p53 (e.g., H1650 cells which areRas^(wt)/p53^(wt)).

Additionally or alternatively, the nucleic acid construct or constructsystem can include a repressor-operator system for control of expressionin cancerous cells having hyperactive cell signaling.

According some embodiments of the invention, the first nucleic acidsequence or the first nucleic acid construct further comprises arepressor of a bacterial repressor-operator system, the repressor beingunder a transcriptional regulation of the cancer-associatedproliferative signaling responsive enhancer element, and wherein thesecond nucleic acid sequence or construct comprises an operator of thebacterial repressor-operator system, such that expression of therepressor inhibits expression of the antitoxin.

According some embodiments of the invention, the repressor comprises theTetracycline repressor (Tet-R) sequence, and wherein the operatorcomprises the tetracycline operator sequence.

According some embodiments of the invention, the operator comprises atleast two repeats of the sequence tetracycline operator sequence.

According some embodiments of the invention, the first nucleic acidsequence or the first nucleic acid construct comprises four repeats ofthe PY2 sequence set forth by SEQ ID NO:2 being upstream and operablylinked to the SV40 minimal promoter region set forth by SEQ ID NO:4, atoxin coding sequence being downstream of and transcriptionallyregulated by the SV40 minimal promoter region, an IRES sequence setforth by SEQ ID NO:7 being downstream and operably linked to the toxincoding sequence, and a Tetracycline repressor set forth by SEQ ID NO: 8being downstream of and operably linked to the IRES sequence.

According some embodiments of the invention, the second nucleic acidsequence or the second nucleic acid construct comprises a CMV minimalpromoter which comprises two repeats of a tetracycline operator as setforth by SEQ ID NO:9 and an antitoxin coding sequence being downstreamof and operably linked to the CMV minimal promoter.

For example, as schematically illustrated in FIG. 2C, in cells withhyperactive RAS, the Tet-R is expressed resulting in its binding to theTetracycline operator and accordingly inhibition of expression of theantitoxin mazE which is downstream to the Tetracycline operator. Incontrast, in cells with wild type RAS the TetR is not overexpressed andaccordingly does not bind to the tetracycline operator, resulting inexpression of the antitoxin maze (FIG. 2D).

It should be noted that when the nucleic acid construct system is used,the first nucleic acid construct and the second nucleic acid constructscan be co-transfected at any MOI ratio into the cell-of-interest (e.g.,the target cell such as the cancerous cell).

According to some embodiments of the invention, the first nucleic acidconstruct and the second nucleic acid constructs are co-transfected at a1:1 MOI ratio.

For example, when using a nucleic acid construct system in which thesecond promoter is stronger than the first promoter, a 1:1 MOI ratio canbe used for co-transfection.

Additionally or alternatively, when using a nucleic acid constructsystem in which the second promoter has a similar ability to directtranscription of a nucleic acid sequence operably linked thereto as theability of the promoter of the first nucleic acid construct, and/or inthe case of using identical promoters in both constructs (e.g., the SV40promoter as shown in FIG. 11A) then the amount of the first nucleic acidconstruct should be higher than the amount of the second nucleic acidconstruct, in order to ensure cell killing.

According to some embodiments of the invention, the first nucleic acidconstruct and the nucleic acid construct are co-transfected into cellsat an MOI ratio of 1 to 0.9, e.g., at an MOI ratio of 1 to 0.8, 1 to0.7, 1 to 0.6, 1 to 0.5, 1 to 0.4, 1 to 0.3, 1 to 0.2, e.g., 1 to 0.1,respectively.

According to some embodiments of the invention, the first nucleic acidconstruct and the nucleic acid construct are co-transfected into cellsat an MOI ratio of 1 to 0.5, respectively.

The nucleic acid construct of some embodiments of the invention includesadditional sequences which render this vector suitable for replicationand integration in prokaryotes, eukaryotes, or preferably both (e.g.,shuttle vectors). In addition, a typical cloning vectors may alsocontain a transcription and translation initiation sequence,transcription and translation terminator and a polyadenylation signal.By way of example, such constructs will typically include a 5′ LTR, atRNA binding site, a packaging signal, an origin of second-strand DNAsynthesis, and a 3′ LTR or a portion thereof.

The nucleic acid construct of some embodiments of the inventiontypically includes a signal sequence for secretion of the peptide from ahost cell in which it is placed. Preferably the signal sequence for thispurpose is a mammalian signal sequence or the signal sequence of thepolypeptide variants of some embodiments of the invention.

Eukaryotic promoters typically contain two types of recognitionsequences, the TATA box and upstream promoter elements. The TATA box,located 25-30 base pairs upstream of the transcription initiation site,is thought to be involved in directing RNA polymerase to begin RNAsynthesis. The other upstream promoter elements determine the rate atwhich transcription is initiated.

Enhancer elements can stimulate transcription up to 1,000 fold fromlinked homologous or heterologous promoters. Enhancers are active whenplaced downstream or upstream from the transcription initiation site.Many enhancer elements derived from viruses have a broad host range andare active in a variety of tissues. For example, the SV40 early geneenhancer is suitable for many cell types. Other enhancer/promotercombinations that are suitable for some embodiments of the inventioninclude those derived from polyoma virus, human or murinecytomegalovirus (CMV), the long term repeat from various retrovirusessuch as murine leukemia virus, murine or Rous sarcoma virus and HIV.See, Enhancers and Eukaryotic Expression, Cold Spring Harbor Press, ColdSpring Harbor, N.Y. 1983, which is incorporated herein by reference.

In the construction of the expression vector, the promoter is preferablypositioned approximately the same distance from the heterologoustranscription start site as it is from the transcription start site inits natural setting. As is known in the art, however, some variation inthis distance can be accommodated without loss of promoter function.

Polyadenylation sequences can also be added to the expression vector inorder to increase the efficiency of toxin or anti-toxin mRNAtranslation. Two distinct sequence elements are required for accurateand efficient polyadenylation: GU or U rich sequences located downstreamfrom the polyadenylation site and a highly conserved sequence of sixnucleotides, AAUAAA, located 11-30 nucleotides upstream. Termination andpolyadenylation signals that are suitable for some embodiments of theinvention include those derived from SV40.

In addition to the elements already described, the nucleic acidconstruct of some embodiments of the invention may typically containother specialized elements intended to increase the level of expressionof cloned nucleic acids or to facilitate the identification of cellsthat carry the recombinant DNA. For example, a number of animal virusescontain DNA sequences that promote the extra chromosomal replication ofthe viral genome in permissive cell types. Plasmids bearing these viralreplicons are replicated episomally as long as the appropriate factorsare provided by genes either carried on the plasmid or with the genomeof the host cell.

The nucleic acid construct may or may not include a eukaryotic replicon.If a eukaryotic replicon is present, then the nucleic acid construct isamplifiable in eukaryotic cells using the appropriate selectable marker.If the nucleic acid construct does not comprise a eukaryotic replicon,no episomal amplification is possible. Instead, the recombinant DNAintegrates into the genome of the engineered cell, where the promoterdirects expression of the desired nucleic acid.

The nucleic acid construct of some embodiments of the invention canfurther include additional polynucleotide sequences that allow, forexample, the translation of several proteins from a single mRNA such asan internal ribosome entry site (IRES) and sequences for genomicintegration of the promoter-chimeric polypeptide.

It will be appreciated that the individual elements comprised in theexpression vector can be arranged in a variety of configurations. Forexample, enhancer elements, promoters and the like, and even thepolynucleotide sequence(s) encoding a toxin or anti-toxin can bearranged in a “head-to-tail” configuration, may be present as aninverted complement, or in a complementary configuration, as ananti-parallel strand. While such variety of configuration is more likelyto occur with non-coding elements of the expression vector, alternativeconfigurations of the coding sequence within the expression vector arealso envisioned.

Examples for mammalian expression vectors include, but are not limitedto, pcDNA3, pcDNA3.1(+/−), pGL3, pZeoSV2(+/−), pSecTag2, pDisplay,pEF/myc/cyto, pCMV/myc/cyto, pCR3.1, pSinRep5, DH26S, DHBB, pNMT1,pNMT41, pNMT81, which are available from Invitrogen, pCI which isavailable from Promega, pMbac, pPbac, pBK-RSV and pBK-CMV which areavailable from Strategene, pTRES which is available from Clontech, andtheir derivatives.

Expression vectors containing regulatory elements from eukaryoticviruses such as retroviruses can be also used. SV40 vectors includepSVT7 and pMT2. Vectors derived from bovine papilloma virus includepBV-1MTHA, and vectors derived from Epstein Bar virus include pHEBO, andp205. Other exemplary vectors include pMSG, pAV009/A⁺, pMTO10/A⁺,pMAMneo-5, baculovirus pDSVE, and any other vector allowing expressionof proteins under the direction of the SV-40 early promoter, SV-40 laterpromoter, metallothionein promoter, murine mammary tumor virus promoter,Rous sarcoma virus promoter, polyhedrin promoter, or other promotersshown effective for expression in eukaryotic cells.

As described above, viruses are very specialized infectious agents thathave evolved, in many cases, to elude host defense mechanisms.Typically, viruses infect and propagate in specific cell types. Thetargeting specificity of viral vectors utilizes its natural specificityto specifically target predetermined cell types and thereby introduce arecombinant gene into the infected cell. Thus, the type of vector usedby some embodiments of the invention will depend on the cell typetransformed. The ability to select suitable vectors according to thecell type transformed is well within the capabilities of the ordinaryskilled artisan and as such no general description of selectionconsideration is provided herein. For example, bone marrow cells can betargeted using the human T cell leukemia virus type I (HTLV-I) andkidney cells may be targeted using the heterologous promoter present inthe baculovirus Autographa californica nucleopolyhedrovirus (AcMNPV) asdescribed in Liang C Y et al., 2004 (Arch Virol. 149: 51-60).

Recombinant viral vectors are useful for in vivo expression oftoxin-anti-toxin protein since they offer advantages such as lateralinfection and targeting specificity. Lateral infection is inherent inthe life cycle of, for example, retrovirus and is the process by which asingle infected cell produces many progeny virions that bud off andinfect neighboring cells. The result is that a large area becomesrapidly infected, most of which was not initially infected by theoriginal viral particles. This is in contrast to vertical-type ofinfection in which the infectious agent spreads only through daughterprogeny. Viral vectors can also be produced that are unable to spreadlaterally. This characteristic can be useful if the desired purpose isto introduce a specified gene into only a localized number of targetedcells.

Various methods can be used to introduce the expression vector of someembodiments of the invention into stem cells. Such methods are generallydescribed in Sambrook et al., Molecular Cloning: A Laboratory Manual,Cold Springs Harbor Laboratory, New York (1989, 1992), in Ausubel etal., Current Protocols in Molecular Biology, John Wiley and Sons,Baltimore, Md. (1989), Chang et al., Somatic Gene Therapy, CRC Press,Ann Arbor, Mich. (1995), Vega et al., Gene Targeting, CRC Press, AnnArbor Mich. (1995), Vectors: A Survey of Molecular Cloning Vectors andTheir Uses, Butterworths, Boston Mass. (1988) and Gilboa et at.[Biotechniques 4 (6): 504-512, 1986] and include, for example, stable ortransient transfection, lipofection, electroporation and infection withrecombinant viral vectors. In addition, see U.S. Pat. Nos. 5,464,764 and5,487,992 for positive-negative selection methods.

Introduction of nucleic acids by viral infection offers severaladvantages over other methods such as lipofection and electroporation,since higher transfection efficiency can be obtained due to theinfectious nature of viruses.

Currently preferred in vivo nucleic acid transfer techniques includetransfection with viral or non-viral constructs, such as adenovirus,lentivirus, Herpes simplex I virus, or adeno-associated virus (AAV) andlipid-based systems. Useful lipids for lipid-mediated transfer of thegene are, for example, DOTMA, DOPE, and DC-Chol [Tonkinson et al.,Cancer Investigation, 14(1): 54-65 (1996)]. The most preferredconstructs for use in gene therapy are viruses, most preferablyadenoviruses, AAV, lentiviruses, or retroviruses. A viral construct suchas a retroviral construct includes at least one transcriptionalpromoter/enhancer or locus-defining element(s), or other elements thatcontrol gene expression by other means such as alternate splicing,nuclear RNA export, or post-translational modification of messenger.Such vector constructs also include a packaging signal, long terminalrepeats (LTRs) or portions thereof, and positive and negative strandprimer binding sites appropriate to the virus used, unless it is alreadypresent in the viral construct. In addition, such a construct typicallyincludes a signal sequence for secretion of the peptide from a host cellin which it is placed. Preferably the signal sequence for this purposeis a mammalian signal sequence or the signal sequence of the polypeptidevariants of some embodiments of the invention. Optionally, the constructmay also include a signal that directs polyadenylation, as well as oneor more restriction sites and a translation termination sequence. By wayof example, such constructs will typically include a 5′ LTR, a tRNAbinding site, a packaging signal, an origin of second-strand DNAsynthesis, and a 3′ LTR or a portion thereof. Other vectors can be usedthat are non-viral, such as cationic lipids, polylysine, and dendrimers.

Other than containing the necessary elements for the transcription andtranslation of the inserted coding sequence, the nucleic acid constructof some embodiments of the invention can also include sequencesengineered to enhance stability, production, purification, yield ortoxicity of the expressed peptide. For example, the expression of afusion protein or a cleavable fusion protein comprising the toxin oranti-toxin protein of some embodiments of the invention and aheterologous protein can be engineered.

According some embodiments of the invention, the nucleic acid constructor the nucleic acid construct system is adeno-virus based.

As used herein the term “adeno-virus” encompasses adenoviruses andadeno-associated virus(es) (AAVs).

Adenoviruses (members of the family Adenoviridae and genusMastadenovirus.) are medium-sized (90-100 nm), nonenveloped (without anouter lipid bilayer) viruses with an icosahedral nucleocapsid at 70-90nm in diameter and each contains a single linear, double-stranded DNAgenome of approximately 36 kb. Within the almost 100 different serotypesof human adenovirus, 51 are known to be pathogenic in humans and tocause a wide range of illnesses, from mild respiratory infections inyoung children (known as the common cold) to life-threateningmulti-organ disease in people with a weakened immune system.

Adeno-associated virus (AAV) is a small virus which infects humans andsome other primate species. AAV belongs to the genus Dependoparvovirus,which in turn belongs to the family Parvoviridae. The virus is a small(20 nm) replication-defective, nonenveloped virus. AAV is not currentlyknown to cause disease. The virus causes a very mild immune response,lending further support to its apparent lack of pathogenicity. Genetherapy vectors using AAV can infect both dividing and quiescent cellsand persist in an extrachromosomal state without integrating into thegenome of the host cell, although in the native virus some integrationof virally carried genes into the host genome does occur. These featuresmake AAV a very attractive candidate for creating viral vectors for genetherapy, and for the creation of isogenic human disease models. Recenthuman clinical trials using AAV for gene therapy in the retina haveshown promise [Maguire A M, et al. (2008). “Safety and Efficacy of GeneTransfer for Leber's Congenital Amaurosis”. New England Journal ofMedicine 358 (21): 2240-8].

There are about 11 AAV serotypes described, all of them can infect cellsfrom multiple diverse tissue types. Tissue specificity is determined bythe capsid serotype and pseudotyping of AAV vectors to alter theirtropism range will likely be important to their use in therapy.

As shown in FIG. 10, the present inventors showed that AAV serotype 6 issuitable for expression of the toxin and anti-toxin proteins using theconstructs of the nucleic acid construct system of some embodiments ofthe invention.

According some embodiments of the invention, the nucleic acid constructor the nucleic acid construct system is Lenti-virus based.

Lentivirus is a genus of viruses of the Retroviridae family,characterized by a long incubation period. Lentiviruses can deliver asignificant amount of viral RNA into the DNA of the host cell and havethe unique ability among retroviruses of being able to infectnon-dividing cells, so they are one of the most efficient methods of agene delivery vector.

The nucleic acid construct or the nucleic acid construct system of someembodiments of the invention can be administered to an organism per se,or in a pharmaceutical composition where it is mixed with suitablecarriers or excipients.

According to an aspect of some embodiments of the invention, there isprovided a pharmaceutical composition comprising the nucleic acidconstruct of some embodiments of the invention or the nucleic acidconstruct system of some embodiments of the invention and apharmaceutically acceptable carrier or diluents.

As used herein a “pharmaceutical composition” refers to a preparation ofone or more of the active ingredients described herein with otherchemical components such as physiologically suitable carriers andexcipients. The purpose of a pharmaceutical composition is to facilitateadministration of a compound to an organism.

Herein the term “active ingredient” refers to the nucleic acid constructof some embodiments of the invention or the nucleic acid constructsystem of some embodiments of the invention accountable for thebiological effect.

Hereinafter, the phrases “physiologically acceptable carrier” and“pharmaceutically acceptable carrier” which may be interchangeably usedrefer to a carrier or a diluent that does not cause significantirritation to an organism and does not abrogate the biological activityand properties of the administered compound. An adjuvant is includedunder these phrases.

Herein the term “excipient” refers to an inert substance added to apharmaceutical composition to further facilitate administration of anactive ingredient. Examples, without limitation, of excipients includecalcium carbonate, calcium phosphate, various sugars and types ofstarch, cellulose derivatives, gelatin, vegetable oils and polyethyleneglycols.

Techniques for formulation and administration of drugs may be found in“Remington's Pharmaceutical Sciences,” Mack Publishing Co., Easton, Pa.,latest edition, which is incorporated herein by reference.

Suitable routes of administration may, for example, include oral,rectal, transmucosal, especially transnasal, intestinal or parenteraldelivery, including intramuscular, subcutaneous and intramedullaryinjections as well as intrathecal, direct intraventricular,intracardiac, e.g., into the right or left ventricular cavity, into thecommon coronary artery, intravenous, intraperitoneal, intranasal, orintraocular injections.

Conventional approaches for drug delivery to the central nervous system(CNS) include: neurosurgical strategies (e.g., intracerebral injectionor intracerebroventricular infusion); molecular manipulation of theagent (e.g., production of a chimeric fusion protein that comprises atransport peptide that has an affinity for an endothelial cell surfacemolecule in combination with an agent that is itself incapable ofcrossing the BBB) in an attempt to exploit one of the endogenoustransport pathways of the BBB; pharmacological strategies designed toincrease the lipid solubility of an agent (e.g., conjugation ofwater-soluble agents to lipid or cholesterol carriers); and thetransitory disruption of the integrity of the BBB by hyperosmoticdisruption (resulting from the infusion of a mannitol solution into thecarotid artery or the use of a biologically active agent such as anangiotensin peptide). However, each of these strategies has limitations,such as the inherent risks associated with an invasive surgicalprocedure, a size limitation imposed by a limitation inherent in theendogenous transport systems, potentially undesirable biological sideeffects associated with the systemic administration of a chimericmolecule comprised of a carrier motif that could be active outside ofthe CNS, and the possible risk of brain damage within regions of thebrain where the BBB is disrupted, which renders it a suboptimal deliverymethod.

Alternately, one may administer the pharmaceutical composition in alocal rather than systemic manner, for example, via injection of thepharmaceutical composition directly into a tissue region of a patient.

The term “tissue” refers to part of an organism consisting of cellsdesigned to perform a function or functions. Examples include, but arenot limited to, brain tissue, retina, skin tissue, hepatic tissue,pancreatic tissue, bone, cartilage, connective tissue, blood tissue,muscle tissue, cardiac tissue brain tissue, vascular tissue, renaltissue, pulmonary tissue, gonadal tissue, hematopoietic tissue.

Pharmaceutical compositions of some embodiments of the invention may bemanufactured by processes well known in the art, e.g., by means ofconventional mixing, dissolving, granulating, dragee-making, levigating,emulsifying, encapsulating, entrapping or lyophilizing processes.

Pharmaceutical compositions for use in accordance with some embodimentsof the invention thus may be formulated in conventional manner using oneor more physiologically acceptable carriers comprising excipients andauxiliaries, which facilitate processing of the active ingredients intopreparations which, can be used pharmaceutically. Proper formulation isdependent upon the route of administration chosen.

For injection, the active ingredients of the pharmaceutical compositionmay be formulated in aqueous solutions, preferably in physiologicallycompatible buffers such as Hank's solution, Ringer's solution, orphysiological salt buffer. For transmucosal administration, penetrantsappropriate to the barrier to be permeated are used in the formulation.Such penetrants are generally known in the art.

For oral administration, the pharmaceutical composition can beformulated readily by combining the active compounds withpharmaceutically acceptable carriers well known in the art. Suchcarriers enable the pharmaceutical composition to be formulated astablets, pills, dragees, capsules, liquids, gels, syrups, slurries,suspensions, and the like, for oral ingestion by a patient.Pharmacological preparations for oral use can be made using a solidexcipient, optionally grinding the resulting mixture, and processing themixture of granules, after adding suitable auxiliaries if desired, toobtain tablets or dragee cores. Suitable excipients are, in particular,fillers such as sugars, including lactose, sucrose, mannitol, orsorbitol; cellulose preparations such as, for example, maize starch,wheat starch, rice starch, potato starch, gelatin, gum tragacanth,methyl cellulose, hydroxypropylmethyl-cellulose, sodiumcarbomethylcellulose; and/or physiologically acceptable polymers such aspolyvinylpyrrolidone (PVP). If desired, disintegrating agents may beadded, such as cross-linked polyvinyl pyrrolidone, agar, or alginic acidor a salt thereof such as sodium alginate.

Dragee cores are provided with suitable coatings. For this purpose,concentrated sugar solutions may be used which may optionally containgum arabic, talc, polyvinyl pyrrolidone, carbopol gel, polyethyleneglycol, titanium dioxide, lacquer solutions and suitable organicsolvents or solvent mixtures. Dyestuffs or pigments may be added to thetablets or dragee coatings for identification or to characterizedifferent combinations of active compound doses.

Pharmaceutical compositions which can be used orally, include push-fitcapsules made of gelatin as well as soft, sealed capsules made ofgelatin and a plasticizer, such as glycerol or sorbitol. The push-fitcapsules may contain the active ingredients in admixture with fillersuch as lactose, binders such as starches, lubricants such as talc ormagnesium stearate and, optionally, stabilizers. In soft capsules, theactive ingredients may be dissolved or suspended in suitable liquids,such as fatty oils, liquid paraffin, or liquid polyethylene glycols. Inaddition, stabilizers may be added. All formulations for oraladministration should be in dosages suitable for the chosen route ofadministration.

For buccal administration, the compositions may take the form of tabletsor lozenges formulated in conventional manner.

For administration by nasal inhalation, the active ingredients for useaccording to some embodiments of the invention are convenientlydelivered in the form of an aerosol spray presentation from apressurized pack or a nebulizer with the use of a suitable propellant,e.g., dichlorodifluoromethane, trichlorofluoromethane,dichloro-tetrafluoroethane or carbon dioxide. In the case of apressurized aerosol, the dosage unit may be determined by providing avalve to deliver a metered amount. Capsules and cartridges of, e.g.,gelatin for use in a dispenser may be formulated containing a powder mixof the compound and a suitable powder base such as lactose or starch.

The pharmaceutical composition described herein may be formulated forparenteral administration, e.g., by bolus injection or continuousinfusion. Formulations for injection may be presented in unit dosageform, e.g., in ampoules or in multidose containers with optionally, anadded preservative. The compositions may be suspensions, solutions oremulsions in oily or aqueous vehicles, and may contain formulatoryagents such as suspending, stabilizing and/or dispersing agents.

Pharmaceutical compositions for parenteral administration includeaqueous solutions of the active preparation in water-soluble form.Additionally, suspensions of the active ingredients may be prepared asappropriate oily or water based injection suspensions. Suitablelipophilic solvents or vehicles include fatty oils such as sesame oil,or synthetic fatty acids esters such as ethyl oleate, triglycerides orliposomes. Aqueous injection suspensions may contain substances, whichincrease the viscosity of the suspension, such as sodium carboxymethylcellulose, sorbitol or dextran. Optionally, the suspension may alsocontain suitable stabilizers or agents which increase the solubility ofthe active ingredients to allow for the preparation of highlyconcentrated solutions.

Alternatively, the active ingredient may be in powder form forconstitution with a suitable vehicle, e.g., sterile, pyrogen-free waterbased solution, before use.

The pharmaceutical composition of some embodiments of the invention mayalso be formulated in rectal compositions such as suppositories orretention enemas, using, e.g., conventional suppository bases such ascocoa butter or other glycerides.

Pharmaceutical compositions suitable for use in context of someembodiments of the invention include compositions wherein the activeingredients are contained in an amount effective to achieve the intendedpurpose. More specifically, a therapeutically effective amount means anamount of active ingredients (the nucleic acid construct of someembodiments of the invention or the nucleic acid construct system ofsome embodiments of the invention) effective to prevent, alleviate orameliorate symptoms of a disorder (e.g., cancer, e.g., colon cancer) orprolong the survival of the subject being treated.

Determination of a therapeutically effective amount is well within thecapability of those skilled in the art, especially in light of thedetailed disclosure provided herein.

For any preparation used in the methods of the invention, thetherapeutically effective amount or dose can be estimated initially fromin vitro and cell culture assays.

For example, a dose can be formulated in animal models to achieve adesired concentration or titer. Such information can be used to moreaccurately determine useful doses in humans.

Toxicity and therapeutic efficacy of the active ingredients describedherein can be determined by standard pharmaceutical procedures in vitro,in cell cultures or experimental animals. The data obtained from thesein vitro and cell culture assays and animal studies can be used informulating a range of dosage for use in human. The dosage may varydepending upon the dosage form employed and the route of administrationutilized. The exact formulation, route of administration and dosage canbe chosen by the individual physician in view of the patient'scondition. (See e.g., Fingl, et al., 1975, in “The Pharmacological Basisof Therapeutics”, Ch. 1 p.1).

Dosage amount and interval may be adjusted individually to providetissue levels of the active ingredient are sufficient to induce orsuppress the biological effect (minimal effective concentration, MEC).The MEC will vary for each preparation, but can be estimated from invitro data. Dosages necessary to achieve the MEC will depend onindividual characteristics and route of administration. Detection assayscan be used to determine plasma concentrations.

Depending on the severity and responsiveness of the condition to betreated, dosing can be of a single or a plurality of administrations,with course of treatment lasting from several days to several weeks oruntil cure is effected or diminution of the disease state is achieved.

The amount of a composition to be administered will, of course, bedependent on the subject being treated, the severity of the affliction,the manner of administration, the judgment of the prescribing physician,etc.

Compositions of some embodiments of the invention may, if desired, bepresented in a pack or dispenser device, such as an FDA approved kit,which may contain one or more unit dosage forms containing the activeingredient. The pack may, for example, comprise metal or plastic foil,such as a blister pack. The pack or dispenser device may be accompaniedby instructions for administration. The pack or dispenser may also beaccommodated by a notice associated with the container in a formprescribed by a governmental agency regulating the manufacture, use orsale of pharmaceuticals, which notice is reflective of approval by theagency of the form of the compositions or human or veterinaryadministration. Such notice, for example, may be of labeling approved bythe U.S. Food and Drug Administration for prescription drugs or of anapproved product insert. Compositions comprising a preparation of theinvention formulated in a compatible pharmaceutical carrier may also beprepared, placed in an appropriate container, and labeled for treatmentof an indicated condition, as is further detailed above.

According to an aspect of some embodiments of the invention there isprovided a method of treating cancer comprising, the method comprisingintroducing into the cancer cells the nucleic acid construct of someembodiments of the invention, or the nucleic acid construct system ofsome embodiments of the invention, wherein the cancer cells arecharacterized by hyper activity of the proliferative signaling ascompared to non-cancerous cells of the same tissue, thereby treating thecancer.

The term “treating” refers to inhibiting, preventing or arresting thedevelopment of a pathology (disease, disorder or condition) and/orcausing the reduction, remission, or regression of a pathology. Those ofskill in the art will understand that various methodologies and assayscan be used to assess the development of a pathology, and similarly,various methodologies and assays may be used to assess the reduction,remission or regression of a pathology.

Thus, the nucleic acid construct of some embodiments of the invention,or the nucleic acid construct system of some embodiments of theinvention is administered to the cancer cells of a subject in needthereof.

As used herein, the term “subject” includes mammals, preferably humanbeings at any age which suffer from the pathology.

According to an aspect of some embodiments of the invention there isprovided a composition comprising the nucleic acid construct of someembodiments of the invention, or the nucleic acid construct system ofany one of some embodiments of the invention for use in treating cancer,wherein cells of the cancer are characterized by hyper activity of theproliferative signaling as compared to non-cancerous cells of the sametissue.

According to some embodiments of the invention, the cancer comprises asolid tumor.

According to some embodiments of the invention, the cancer comprisescancer metastases and/or cancer micrometastases.

According to some embodiments of the invention, the cancer comprisescancer micrometastases.

Non-limiting examples of the cancer which can be treated by thecomposition (e.g., the nucleic acid construct or the nucleic acidconstruct system) or the method of some embodiments of the inventioninclude any solid or non-solid cancer and/or cancer metastasis,including, but is not limiting to, tumors of the gastrointestinal tract(colon carcinoma, rectal carcinoma, colorectal carcinoma, colorectalcancer, colorectal adenoma, hereditary nonpolyposis type 1, hereditarynonpolyposis type 2, hereditary nonpolyposis type 3, hereditarynonpolyposis type 6; colorectal cancer, hereditary nonpolyposis type 7,small and/or large bowel carcinoma, esophageal carcinoma, tylosis withesophageal cancer, stomach carcinoma, pancreatic carcinoma, pancreaticendocrine tumors), endometrial carcinoma, dermatofibrosarcomaprotuberans, gallbladder carcinoma, Biliary tract tumors, prostatecancer, prostate adenocarcinoma, renal cancer (e.g., Wilms' tumor type 2or type 1), liver cancer (e.g., hepatoblastoma, hepatocellularcarcinoma, hepatocellular cancer), bladder cancer, embryonalrhabdomyosarcoma, germ cell tumor, trophoblastic tumor, testicular germcells tumor, immature teratoma of ovary, uterine, epithelial ovarian,sacrococcygeal tumor, choriocarcinoma, placental site trophoblastictumor, epithelial adult tumor, ovarian carcinoma, serous ovarian cancer,ovarian sex cord tumors, cervical carcinoma, uterine cervix carcinoma,small-cell and non-small cell lung carcinoma, nasopharyngeal, breastcarcinoma (e.g., ductal breast cancer, invasive intraductal breastcancer, sporadic; breast cancer, susceptibility to breast cancer, type 4breast cancer, breast cancer-1, breast cancer-3; breast-ovarian cancer),squamous cell carcinoma (e.g., in head and neck), neurogenic tumor,astrocytoma, ganglioblastoma, neuroblastoma, lymphomas (e.g., Hodgkin'sdisease, non-Hodgkin's lymphoma, B cell, Burkitt, cutaneous T cell,histiocytic, lymphoblastic, T cell, thymic), gliomas, adenocarcinoma,adrenal tumor, hereditary adrenocortical carcinoma, brain malignancy(tumor), various other carcinomas (e.g., bronchogenic large cell,ductal, Ehrlich-Lettre ascites, epidermoid, large cell, Lewis lung,medullary, mucoepidermoid, oat cell, small cell, spindle cell,spinocellular, transitional cell, undifferentiated, carcinosarcoma,choriocarcinoma, cystadenocarcinoma), ependimoblastoma, epithelioma,erythroleukemia (e.g., Friend, lymphoblast), fibrosarcoma, giant celltumor, glial tumor, glioblastoma (e.g., multiforme, astrocytoma), gliomahepatoma, heterohybridoma, heteromyeloma, histiocytoma, hybridoma (e.g.,B cell), hypernephroma, insulinoma, islet tumor, keratoma,leiomyoblastoma, leiomyosarcoma, leukemia (e.g., acute lymphatic, acutelymphoblastic, acute lymphoblastic pre-B cell, acute lymphoblastic Tcell leukemia, acute—megakaryoblastic, monocytic, acute myelogenous,acute myeloid, acute myeloid with eosinophilia, B cell, basophilic,chronic myeloid, chronic, B cell, eosinophilic, Friend, granulocytic ormyelocytic, hairy cell, lymphocytic, megakaryoblastic, monocytic,monocytic-macrophage, myeloblastic, myeloid, myelomonocytic, plasmacell, pre-B cell, promyelocytic, subacute, T cell, lymphoid neoplasm,predisposition to myeloid malignancy, acute nonlymphocytic leukemia),lymphosarcoma, melanoma, mammary tumor, mastocytoma, medulloblastoma,mesothelioma, metastatic tumor, monocyte tumor, multiple myeloma,myelodysplastic syndrome, myeloma, nephroblastoma, nervous tissue glialtumor, nervous tissue neuronal tumor, neurinoma, neuroblastoma,oligodendroglioma, osteochondroma, osteomyeloma, osteosarcoma (e.g.,Ewing's), papilloma, transitional cell, pheochromocytoma, pituitarytumor (invasive), plasmacytoma, retinoblastoma, rhabdomyosarcoma,sarcoma (e.g., Ewing's, histiocytic cell, Jensen, osteogenic, reticulumcell), schwannoma, subcutaneous tumor, teratocarcinoma (e.g.,pluripotent), teratoma, testicular tumor, thymoma and trichoepithelioma,gastric cancer, fibrosarcoma, glioblastoma multiforme; multiple glomustumors, Li-Fraumeni syndrome, liposarcoma, lynch cancer family syndromeII, male germ cell tumor, mast cell leukemia, medullary thyroid,multiple meningioma, endocrine neoplasia myxosarcoma, paraganglioma,familial nonchromaffin, pilomatricoma, papillary, familial and sporadic,rhabdoid predisposition syndrome, familial, rhabdoid tumors, soft tissuesarcoma, and Turcot syndrome with glioblastoma.

According to some embodiments of the invention, the cancer comprisescolon cancer.

According to some embodiments of the invention, the cancer compriseslung cancer.

According to some embodiments of the invention, the cancer comprisespancreatic cancer.

According to some embodiments of the invention, the cancer comprisesgastric cancer.

According to some embodiments of the invention, the cancer ischaracterized by a hyperactive RAS GTPase activity.

It should be noted that in cells where the RAS is hyperactive thehydrolysis of GTP (Guanosine-5′-triphosphate) to GDP (Guanosinediphosphate) is prevented, thus the ras pathway is always on (active)and is not deactivated, since it binds to GTP.

According to some embodiments of the invention, the RAS is a KRASprotein and wherein the hyperactive KRAS is caused by a G13D mutation inthe KRAS protein set forth by SEQ ID NO:16 (i.e., substitution ofglycine with aspartic acid at amino acid position 13 in the KRAS proteinset forth by SEQ ID NO:16).

According to some embodiments of the invention, the RAS is an NRASprotein and wherein the hyperactive NRAS is caused by a Q61K mutation inthe NRAS protein set forth by SEQ ID NO:17 (i.e., substitution ofGlutamine with Lysine at amino acid position 61 in the NRAS protein setforth by SEQ ID NO:17).

According to some embodiments of the invention, the RAS is a HRASprotein and wherein the hyperactive HRAS is caused by a G12V mutation inthe HRAS protein set forth by SEQ ID NO:18 (i.e., substitution ofGlycine with Valine at amino acid position 12 of the HRAS protein setforth by SEQ ID NO:18).

According to some embodiments of the invention, the active agentsdescribed hereinabove (e.g., the nucleic acid construct of someembodiments of the invention and/or the nucleic acid construct system ofsome embodiments of the invention) can be provided to the subject inneed thereof along with an additional medicament identified for treatingthe cancer, i.e., by combination therapy.

Therapeutic regimen for treatment of cancer suitable for combinationwith the nucleic acid construct of some embodiments of the inventionand/or the nucleic acid construct system of some embodiments of theinvention include, but are not limited to chemotherapy, biologicaltherapy, immunological therapy, radiotherapy, phototherapy andphotodynamic therapy, surgery, nutritional therapy, ablative therapy,combined radiotherapy and chemotherapy, brachiotherapy, proton beamtherapy, immunotherapy, cellular therapy and photon beam radiosurgicaltherapy.

According to some embodiments of the invention, the method furthercomprising treating a subject having the cancer by a treatment selectedfrom the group consisting of: chemotherapy, biological therapy,radiotherapy, phototherapy, photodynamic therapy, surgery, nutritionaltherapy, ablative therapy, combined radiotherapy and chemotherapy,brachiotherapy, proton beam therapy, immunotherapy, cellular therapy andphoton beam radiosurgical therapy.

According to some embodiments of the invention, the composition furthercomprises an agent suitable for a treatment selected from the groupconsisting of: chemotherapy, biological therapy, photodynamic therapy,nutritional therapy, brachiotherapy, immunotherapy, and cellulartherapy.

It should be noted that such synergistic activity of treatment with thenucleic acid construct of some embodiments of the invention and/or thenucleic acid construct system of some embodiments of the inventiontreatment with additional therapeutic methods or compositions has thepotential to significantly reduce the effective clinical doses of suchtreatments, thereby reducing the often devastating negative side effectsand high cost of the treatment.

Anti-cancer drugs that can be co-administered with the compounds of theinvention include, but are not limited to Acivicin; Aclarubicin;Acodazole Hydrochloride; Acronine; Adriamycin; Adozelesin; Aldesleukin;Altretamine; Ambomycin; Ametantrone Acetate; Aminoglutethimide;Amsacrine; Anastrozole; Anthramycin; Asparaginase; Asperlin;Azacitidine; Azetepa; Azotomycin; Batimastat; Benzodepa; Bicalutamide;Bisantrene Hydrochloride; Bisnafide Dimesylate; Bizelesin; BleomycinSulfate; Brequinar Sodium; Bropirimine; Busulfan; Cactinomycin;Calusterone; Caracemide; Carbetimer; Carboplatin; Carmustine; CarubicinHydrochloride; Carzelesin; Cedefingol; Chlorambucil; Cirolemycin;Cisplatin; Cladribine; Crisnatol Mesylate; Cyclophosphamide; Cytarabine;Dacarbazine; Dactinomycin; Daunorubicin Hydrochloride; Decitabine;Dexormaplatin; Dezaguanine; Dezaguanine Mesylate; Diaziquone; Docetaxel;Doxorubicin; Doxorubicin Hydrochloride; Droloxifene; DroloxifeneCitrate; Dromostanolone Propionate; Duazomycin; Edatrexate; EflornithineHydrochloride; Elsamitrucin; Enloplatin; Enpromate; Epipropidine;Epirubicin Hydrochloride; Erbulozole; Esorubicin Hydrochloride;Estramustine; Estramustine Phosphate Sodium; Etanidazole; Etoposide;Etoposide Phosphate; Etoprine; Fadrozole Hydrochloride; Fazarabine;Fenretinide; Floxuridine; Fludarabine Phosphate; Fluorouracil;Flurocitabine; Fosquidone; Fostriecin Sodium; Gemcitabine; GemcitabineHydrochloride; Hydroxyurea; Idarubicin Hydrochloride; Ifosfamide;Ilmofosine; Interferon Alfa-2a; Interferon Alfa-2b; Interferon Alfa-n1;Interferon Alfa-n3; Interferon Beta-I a; Interferon Gamma-I b;Iproplatin; Irinotecan Hydrochloride; Lanreotide Acetate; Letrozole;Leuprolide Acetate; Liarozole Hydrochloride; Lometrexol Sodium;Lomustine; Losoxantrone Hydrochloride; Masoprocol; Maytansine;Mechlorethamine Hydrochloride; Megestrol Acetate; Melengestrol Acetate;Melphalan; Menogaril; Mercaptopurine; Methotrexate; Methotrexate Sodium;Metoprine; Meturedepa; Mitindomide; Mitocarcin; Mitocromin; Mitogillin;Mitomalcin; Mitomycin; Mitosper; Mitotane; Mitoxantrone Hydrochloride;Mycophenolic Acid; Nocodazole; Nogalamycin; Ormaplatin; Oxisuran;Paclitaxel; Pegaspargase; Peliomycin; Pentamustine; Peplomycin Sulfate;Perfosfamide; Pipobroman; Piposulfan; Piroxantrone Hydrochloride;Plicamycin; Plomestane; Porfimer Sodium; Porfiromycin; Prednimustine;Procarbazine Hydrochloride; Puromycin; Puromycin Hydrochloride;Pyrazofurin; Riboprine; Rogletimide; Safingol; Safingol Hydrochloride;Semustine; Simtrazene; Sparfosate Sodium; Sparsomycin; SpirogermaniumHydrochloride; Spiromustine; Spiroplatin; Streptonigrin; Streptozocin;Sulofenur; Talisomycin; Taxol; Tecogalan Sodium; Tegafur; TeloxantroneHydrochloride; Temoporfin; Teniposide; Teroxirone; Testolactone;Thiamiprine; Thioguanine; Thiotepa; Tiazofuirin; Tirapazamine; TopotecanHydrochloride; Toremifene Citrate; Trestolone Acetate; TriciribinePhosphate; Trimetrexate; Trimetrexate Glucuronate; Triptorelin;Tubulozole Hydrochloride; Uracil Mustard; Uredepa; Vapreotide;Verteporfin; Vinblastine Sulfate; Vincristine Sulfate; Vindesine;Vindesine Sulfate; Vinepidine Sulfate; Vinglycinate Sulfate;Vinleurosine Sulfate; Vinorelbine Tartrate; Vinrosidine Sulfate;Vinzolidine Sulfate; Vorozole; Zeniplatin; Zinostatin; ZorubicinHydrochloride. Additional antineoplastic agents include those disclosedin Chapter 52, Antineoplastic Agents (Paul Calabresi and Bruce A.Chabner), and the introduction thereto, 1202-1263, of Goodman andGilman's “The Pharmacological Basis of Therapeutics”, Eighth Edition,1990, McGraw-Hill, Inc. (Health Professions Division).

Approved chemotherapy include, but are not limited to, abarelix,aldesleukin, aldesleukin, alemtuzumab, alitretinoin, allopurinol,altretamine, amifostine, anastrozole, arsenic trioxide, asparaginase,azacitidine, bevacuzimab, bexarotene, bleomycin, bortezomib, busulfan,calusterone, capecitabine, carboplatin, carmustine, celecoxib,cetuximab, cisplatin, cladribine, clofarabine, cyclophosphamide,cytarabine, dacarbazine, dactinomycin, actinomycin D, Darbepoetin alfa,Darbepoetin alfa, daunorubicin liposomal, daunorubicin, decitabine,Denileukin diftitox, dexrazoxane, dexrazoxane, docetaxel, doxorubicin,dromostanolone propionate, Elliott's B Solution, epirubicin, Epoetinalfa, erlotinib, estramustine, etoposide, exemestane, Filgrastim,floxuridine, fludarabine, fluorouracil 5-FU, fulvestrant, gefitinib,gemcitabine, gemtuzumab ozogamicin, goserelin acetate, histrelinacetate, hydroxyurea, Ibritumomab Tiuxetan, idarubicin, ifosfamide,imatinib mesylate, interferon alfa 2a, Interferon alfa-2b, irinotecan,lenalidomide, letrozole, leucovorin, Leuprolide Acetate, levamisole,lomustine, CCNU, meclorethamine, nitrogen mustard, megestrol acetate,melphalan, L-PAM, mercaptopurine 6-MP, mesna, methotrexate, mitomycin C,mitotane, mitoxantrone, nandrolone phenpropionate, nelarabine,Nofetumomab, Oprelvekin, Oprelvekin, oxaliplatin, paclitaxel,palifermin, pamidronate, pegademase, pegaspargase, Pegfilgrastim,pemetrexed disodium, pentostatin, pipobroman, plicamycin mithramycin,porfimer sodium, procarbazine, quinacrine, Rasburicase, Rituximab,sargramostim, sorafenib, streptozocin, sunitinib maleate, tamoxifen,temozolomide, teniposide VM-26, testolactone, thioguanine 6-TG,thiotepa, thiotepa, topotecan, toremifene, Tositumomab, Trastuzumab,tretinoin ATRA, Uracil Mustard, valrubicin, vinblastine, vinorelbine,zoledronate and zoledronic acid.

Anti-cancer biological drugs that can be co-administered with thecompounds of the invention include, but are not limited to bevacizumab(AVASTIN™ Genentech Inc.), Cetuximab (ERBITUX™ ImClone SystemsIncorporated), Panitumumab (VECTIBIX™ Immunex Corporation) and/or anycombination thereof.

Additional anti-cancer drugs that can be co-administered with thecompounds of the invention include, but are not limited to 5-FU,Capecitabine (XELODA™ Hoffmann-La Roche, Inc), Irinotecan (CAMPTOSAR™YAKULT HONSHA COMPANY, LTD), Oxaliplatin (ELOXATIN™ Sanofi),Trifluridine and tipiracil (LONSURF™ TAIHO PHARMACEUTICAL CO., LTD.),Gemcitabine (GEMZAR™ Eli Lilly and Company), Albumin-bound paclitaxel(ABRAXANE™ of ABRAXIS BIOSCIENCE, LLC), Cisplatin,Paclitaxel (TAXOL™Bristol-Myers Squibb Company), Docetaxel (TAXOTERE™ AVENTIS PHARMAS.A.), Irinotecan liposome (ONIVYDE™ Merrimack Pharmaceuticals, Inc.),dacarbazine (DTIC-DOME™ BAYER HEALTHCARE PHARMACEUTICALS INC.),ETOPOSIDE (ETOPOPHOS™ Bristol-Myers Squibb Company), Temozolomide(TEMODAL™ Schering Corporation), lapatinib (Tyverb™ GlaxoSmithKline),erlotinib (Tarceva™ Astellas Pharma Inc.), everolimus (AFINITOR™Novartis AG), and/or any combination thereof. It should be noted thatany combination of known anti0cancer treatment (e.g., biological,immunological, chemotherapy and the like) can be combined with the genetherapy approach of the claimed compositions

As used herein the term “about” refers to ±10%.

The terms “comprises”, “comprising”, “includes”, “including”, “having”and their conjugates mean “including but not limited to”.

The term “consisting of” means “including and limited to”.

The term “consisting essentially of” means that the composition, methodor structure may include additional ingredients, steps and/or parts, butonly if the additional ingredients, steps and/or parts do not materiallyalter the basic and novel characteristics of the claimed composition,method or structure.

As used herein, the singular form “a”, “an” and “the” include pluralreferences unless the context clearly dictates otherwise. For example,the term “a compound” or “at least one compound” may include a pluralityof compounds, including mixtures thereof.

Throughout this application, various embodiments of this invention maybe presented in a range format. It should be understood that thedescription in range format is merely for convenience and brevity andshould not be construed as an inflexible limitation on the scope of theinvention. Accordingly, the description of a range should be consideredto have specifically disclosed all the possible subranges as well asindividual numerical values within that range. For example, descriptionof a range such as from 1 to 6 should be considered to have specificallydisclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numberswithin that range, for example, 1, 2, 3, 4, 5, and 6. This appliesregardless of the breadth of the range.

Whenever a numerical range is indicated herein, it is meant to includeany cited numeral (fractional or integral) within the indicated range.The phrases “ranging/ranges between” a first indicate number and asecond indicate number and “ranging/ranges from” a first indicate number“to” a second indicate number are used herein interchangeably and aremeant to include the first and second indicated numbers and all thefractional and integral numerals therebetween.

As used herein the term “method” refers to manners, means, techniquesand procedures for accomplishing a given task including, but not limitedto, those manners, means, techniques and procedures either known to, orreadily developed from known manners, means, techniques and proceduresby practitioners of the chemical, pharmacological, biological,biochemical and medical arts.

When reference is made to particular sequence listings, such referenceis to be understood to also encompass sequences that substantiallycorrespond to its complementary sequence as including minor sequencevariations, resulting from, e.g., sequencing errors, cloning errors, orother alterations resulting in base substitution, base deletion or baseaddition, provided that the frequency of such variations is less than 1in 50 nucleotides, alternatively, less than 1 in 100 nucleotides,alternatively, less than 1 in 200 nucleotides, alternatively, less than1 in 500 nucleotides, alternatively, less than 1 in 1000 nucleotides,alternatively, less than 1 in 5,000 nucleotides, alternatively, lessthan 1 in 10,000 nucleotides.

It is understood that any Sequence Identification Number (SEQ ID NO)disclosed in the instant application can refer to either a DNA sequenceor a RNA sequence, depending on the context where that SEQ ID NO ismentioned, even if that SEQ ID NO is expressed only in a DNA sequenceformat or a RNA sequence format. For example, SEQ ID NO:6 is expressedin a DNA sequence format (e.g., reciting T for thymine), but it canrefer to either a DNA sequence that corresponds to an E. coli MazFribonuclease nucleic acid sequence, or the RNA sequence of an RNAmolecule nucleic acid sequence. Similarly, though some sequences areexpressed in a RNA sequence format (e.g., reciting U for uracil),depending on the actual type of molecule being described, it can referto either the sequence of a RNA molecule comprising a dsRNA, or thesequence of a DNA molecule that corresponds to the RNA sequence shown.In any event, both DNA and RNA molecules having the sequences disclosedwith any substitutes are envisioned.

It is appreciated that certain features of the invention, which are, forclarity, described in the context of separate embodiments, may also beprovided in combination in a single embodiment. Conversely, variousfeatures of the invention, which are, for brevity, described in thecontext of a single embodiment, may also be provided separately or inany suitable subcombination or as suitable in any other describedembodiment of the invention. Certain features described in the contextof various embodiments are not to be considered essential features ofthose embodiments, unless the embodiment is inoperative without thoseelements.

Various embodiments and aspects of the present invention as delineatedhereinabove and as claimed in the claims section below find experimentalsupport in the following examples.

EXAMPLES

Reference is now made to the following examples, which together with theabove descriptions illustrate some embodiments of the invention in a nonlimiting fashion.

Generally, the nomenclature used herein and the laboratory proceduresutilized in the present invention include molecular, biochemical,microbiological and recombinant DNA techniques. Such techniques arethoroughly explained in the literature. See, for example, “MolecularCloning: A laboratory Manual” Sambrook et al., (1989); “CurrentProtocols in Molecular Biology” Volumes I-III Ausubel, R. M., ed.(1994); Ausubel et al., “Current Protocols in Molecular Biology”, JohnWiley and Sons, Baltimore, Md. (1989); Perbal, “A Practical Guide toMolecular Cloning”, John Wiley & Sons, New York (1988); Watson et al.,“Recombinant DNA”, Scientific American Books, New York; Birren et al.(eds) “Genome Analysis: A Laboratory Manual Series”, Vols. 1-4, ColdSpring Harbor Laboratory Press, New York (1998); methodologies as setforth in U.S. Pat. Nos. 4,666,828; 4,683,202; 4,801,531; 5,192,659 and5,272,057; “Cell Biology: A Laboratory Handbook”, Volumes I-III Cellis,J. E., ed. (1994); “Current Protocols in Immunology” Volumes I-IIIColigan J. E., ed. (1994); Stites et al. (eds), “Basic and ClinicalImmunology” (8th Edition), Appleton & Lange, Norwalk, Conn. (1994);Mishell and Shiigi (eds), “Selected Methods in Cellular Immunology”, W.H. Freeman and Co., New York (1980); available immunoassays areextensively described in the patent and scientific literature, see, forexample, U.S. Pat. Nos. 3,791,932; 3,839,153; 3,850,752; 3,850,578;3,853,987; 3,867,517; 3,879,262; 3,901,654; 3,935,074; 3,984,533;3,996,345; 4,034,074; 4,098,876; 4,879,219; 5,011,771 and 5,281,521;“Oligonucleotide Synthesis” Gait, M. J., ed. (1984); “Nucleic AcidHybridization” Hames, B. D., and Higgins S. J., eds. (1985);“Transcription and Translation” Hames, B. D., and Higgins S. J., Eds.(1984); “Animal Cell Culture” Freshney, R. I., ed. (1986); “ImmobilizedCells and Enzymes” IRL Press, (1986); “A Practical Guide to MolecularCloning” Perbal, B., (1984) and “Methods in Enzymology” Vol. 1-317,Academic Press; “PCR Protocols: A Guide To Methods And Applications”,Academic Press, San Diego, Calif. (1990); Marshak et al., “Strategiesfor Protein Purification and Characterization—A Laboratory CourseManual” CSHL Press (1996); all of which are incorporated by reference asif fully set forth herein. Other general references are providedthroughout this document. The procedures therein are believed to be wellknown in the art and are provided for the convenience of the reader. Allthe information contained therein is incorporated herein by reference.

General Materials and Experimental Methods

Reagents—All reagents were purchased from Sigma, Israel unless otherwisestated. All secondary H1RP-conjugated antibodies were from JacksonImmunoResearch Laboratories, USA. ECL reagent, cell culture media andadditives were from Beit-Haemek, Israel. Nitrocellulose filters werefrom Schleicher & Schuell BioScience, USA. Annexin V and Reddot2 dyewere purchased from Biotium, and G418 was purchased from Gibco. Allplasmid and DNA fragment purifications were carried out with aHigh-Speed Plasmid Mini Kit and a Zymoclean™ Gel/PCR DNA recovery Kit(Fermentas and Zemo Research, respectively) unless otherwise specified.T4 DNA ligase and restriction enzymes were purchased from New EnglandBiolabs, USA. DNA ligations were carried out overnight at 16° C.

Bacterial strains—The following Escherichia coli (E. coli) strains wereused: DH5a (Stratagene, USA) for plasmid propagation and BJ5183(Stratagene, USA) for the generation of recombinant adenovirus plasmidDNA.

Cell lines—HT29 human colon adenocarcinoma, HCT116 human colon cancer,R1 KRAS transformed rat enterocytes and HEK293 human kidney cell lineswere grown in high-glucose Dulbecco's modified Eagle's medium (DMEM),all supplemented with 5% heat-inactivated fetal bovine serum (FBS), 1%penicillin and streptomycin in an atmosphere of 95% oxygen and 5% CO₂. 1μg/ml tetracycline was added to the HEK293 medium for TA virusproduction. In addition, 600 μg/ml G418 was added to the culture mediumof MazE-expressing cells.

Oligonucleotides—All the oligonucleotides that were used in this studywere purchased from Sigma, Israel.

Recombinant DNA techniques were carried out according to standardprotocols or as recommended by the manufacturers. A more detaileddescription of the procedure is provided hereinbelow.

Construction and Propagation of Recombinant Adenovirus Vectors

Construction of the vector encoding for “mCherry-MazF”—The monomeric redfluorescence protein mCherry was amplified from an expression cassetteby PCR. The PCR product was digested with HindIII and XbaI and clonedbetween the corresponding sites of the plasmid “pGL3 promoter-Py4-PUMA”(replacing the PUMA gene) that had been previously prepared, generatingthe “pGLl3 promoter-Py4-mCherry” plasmid.

The MazF coding sequence which was amplified from an expression cassette(kindly provided by Dr. Assaf Shapira, Department of MolecularMicrobiology and Biotechnology, Tel-Aviv University, Israel) using theprimes Hind-Cher-For and Xba-Maz-Rev[5′-CTTTTGCAAAAAGCTTCCACCATGggaattcacGTGAGCAAGGGCGAGGAGG-3′ (SEQ IDNO:21) and 5′-CCGCCCCGACTCTAGActaaccggtgccaatcagtacgttaattttggc-3′ (SEQID NO:22), respectively] and was fused to the C terminus of the mCherryunder the RRE, Py4-SV40 mP, generating the “pGL3promoter-Py4-mCherry-MazF” plasmid. This intermediate vector was used asthe template for the amplification of the Py4-mCherry-MazF fusion genes,and the amplified product was cloned using the AdEasy system (pShuttleand pAdEasy-1), as previously described (He et al., 1998, Luo et al.,2007) to generate the regulated expression cassette“pAdEasy-Py4-mCherry-MazF”.

Construction of the vector encoding for “mCherry”—The sequence of thered fluorescent protein mCherry3 was amplified by PCR from an expressioncassette and cloned downstream to a CMV promoter, generating theexpression cassette “pAdEasy-CMV-mCherry”.

Construction of the vector encoding for “MazEF”—The tetracyclinerepressor coding sequence was located downstream to an IRES sequence andcloned downstream to the SV40-mCherry-MazF sequence. An additional armwas introduced to this cassette, controlled by a different minimalpromoter (mP) without the RAS-responsive elements. Starting from its Nterminus, this arm includes: a CMV mP, a tetracycline operator sequence,the full MazE coding sequence, IRES and EGFP coding sequence (FIG. 2,A),generating the expression cassette“pAdEasy-Py4-mCherry-MazF-IRES-TetR-CMVmp-MazE-IRES-EGFP”.

Plasmids were isolated by a standard miniprep procedure and sequenced toconfirm their predicted composition. For the production of virusparticles, the plasmids “pAdEasy-Py4-mCherry-MazF”,“pAdEasy-Py4-mCherry-MazF-IRES-TetR-CMVmp-MazE-IRES-EGFP”,“pAdEasy-SV40-mCherry-MazF-IRES-TetR-CMVmp-MazE-IRES-EGFP” and“pAdEasy-mCherry” were isolated from selected “positive” clones anddigested with PacI. Next, DNA was purified using a Zymoclean™ Gel DNARecovery Kit according to the manufacturer's instructions. Fivemicrograms (μg) of the purified, digested plasmids were used totransfect 70% confluence HEK293 cells (supplemented with tetracycline ata final concentration of 1 μg/ml when the TA construct had beentransfected) and HEK293-MazE cells (for the mCherry-MazF construct,supplement S2) in 60-mm culture plates using the calcium-phosphatemethod. After 24 hours, the transfection medium was replaced with 5 mlof fresh medium (that was supplemented with 1 μg/ml of tetracycline inthe TA system). From 7 to 10 days post-transfection, when a cytopathiceffect (CPE) had been clearly observed, the cells were collected byscraping them off the plate and pelleting them along with any floatingcells in the culture. The pellet was washed once with phosphate-bufferedsaline (PBS), suspended in 0.5 ml PBS and subjected to 4 cycles offreeze/thaw. Cell debris was precipitated by brief centrifugation, and300 μl of the supernatant that contained virus particles were used toinfect 70% confluent HEK293 and its derivative cells in 10 cm plates(first amplification “cycle”). When one-third to one-half of the cellshad been detached (usually after 3-5 days), virus particles werereleased by freeze/thaw cycles as described above. The supernatantcontaining viruses was kept at −80° C.

High scale production of the adenoviruses was performed by SIRIONbiotech, Germany.

Establishment of new packaging cell line for the production of thetoxin—The lethal transgene MazF coding sequence was fused to the mCherrygene and cloned into the Ad5 adenoviral vector plasmid DNA. Thepropagation of the virus particles in the HEK293 packaging cellsresulted in low yields of virus production due to the highly toxicnature of the selected gene. Hence, an innovative packaging system wasestablished, based on MazF-resistant HEK293 cells that constitutivelyexpress the MazE antitoxin, encoded from the pIRES2-MazE-IRES-EGFPplasmid. The EGFP marker was used for effective and rapid screening ofstable clones according to their high fluorescence intensity. The yieldwas higher by almost 2 orders of magnitude after propagation inMazF-resistant cells compared to propagation in parental HEK293packaging cells.

Cell-viability assay—The cell-killing activities of adenovirusesencoding for MazF, and MazEF were measured by the Thiazolyl BlueTetrazoliam Bromide (MTT) enzymatic assay. Briefly, 1×10⁴ cells wereseeded in 96-well plates. After 24 hours, different dilutions ofrecombinant adenoviruses encoding for the above-described cassettes wereadded. At 72 hours post infection, the media was replaced by fresh media(100 μl per well) containing 1 mg/ml MTT and the cells were incubatedfor 2-4 more hours. MTT-formazan crystals were dissolved by the additionof extraction solution (0.1N HCl in absolute isopropanol). Absorbance at570 nm and a reference wavelength of 690 nm was recorded on an automatedmicroplate reader.

Detection of Cell Death

Apoptosis—Cells were seeded in 12-well plates (1×10⁵ cells/well) incomplete medium and infected with the different adenoviruses at severalmultiple of infection (MOI) for 72 hours. Annexin V (Annexin V, CF640Rconjugate) was detected according to the manufacturer's protocol(Biotium Inc., USA). The cells were washed with PBS and then incubatedin a solution of Annexin V binding protein. The cells were analyzed byflow cytometry [FACSCalibur (Becton Dickinson, CA)], and the resultswere analyzed with the CELLQuest program (Becton Dickinson).

Total dead cells—Cells were seeded in 12-well plates (1×10⁵ cells/well)in complete medium and infected with the different adenoviruses atseveral MOI for 72 hours. Dead cells were detected by RedDotTM2, afar-red cell membrane-impermeable nuclear dye, according to themanufacturer's protocol (Biotium Inc., USA). The cells were washed withPBS, and then incubated in a solution of RedDot2 dye. Far red nuclearstaining was detected by flow cytometry.

End-Point Dilution Assay (EPDA)—1×10⁴ HEK293 cells/well were seeded in96-well plate in 100 μl of growth medium. The recombinant adenovirusstock solutions were serially diluted 10-fold to a concentration in arange of 10⁻³-10⁻¹⁰ into growth medium and added to each well in columns1-10. Virus-free growth medium was added to the wells in columns 11 and12 which served as controls for the viability of non-infected cells. Theplate was incubated in a humidified CO₂ (5%) incubator for 10 days at37° C. Each well was checked for CPE using a microscope. A well wasscored as CPE positive even if only a few cells showed cytopathiceffects. The viral titer was calculated according to the formula: Titer(pfu/ml)=10^((x+0.8)), where x=the sum of the fractions of CPE-positivewells for each dilution (10 out of 10 wells with CPE calculated as “1”).

Colony formation assay—5×10⁵ HCT116 and HT29 cells were seeded per wellin 6-well plates. After 24 hours, the cells were infected with 25 and 10MOI of the viruses“pAdEasy-Py4-mCherry-MazF-IRES-TetR-CMVmp-MazE-IRES-EGFP” and“pAdEasy-SV40-mCherry-MazF-IRES-TetR-CMVmp-MazE-IRES-EGFP” or leftun-infected. After 7 hours, the cells were trypsinized and seeded in3-fold dilutions and incubated for 7 days. Surviving colonies were fixedwith 4% formaldehyde in PBS and stained with 0.02% crystal violet.

Xenograft model in mice for measuring in vivo tumor development—Male 6-8week old athymic nude mice (Harlan Laboratories) (n=18) were housed insterile cages and handled with aseptic precautions. The mice were fed adlibitum. For testing the therapeutic potential of the TA system,exponentially growing HCT116 cells were harvested and resuspended at afinal concentration of 5×10⁶ cells per 0.1 ml PBS per injection. Thecells were injected subcutaneously at two sites on the backs of themice. When tumors were palpable (˜0.3-0.5 cm³), the mice were randomlydivided into three groups of six each and the treatment was started. Theviruses Ad-Py4-TA (6 mice) and ΔPy4-TA 1×10⁹ pfu (6 mice) or PBS (6mice) were administrated via two intraperitoneal injections with a 3-dayinterval between injections. The mice were weighed, the tumor volume wasmeasured with a caliper every two days starting from treatment onset andthose results were carefully plotted. Tumor volume was calculated as4/3π·a·b². At the end of the experiment, MazF and MazE expression in thetumors was monitored by imaging using the CRi Maestro system. The micewere anesthetized and then sacrificed by cervical dislocation and thetumors were excised.

TABLE 2 Table 2 RT-PCR primers SEQ SEQ Forward primer ID Reverse primerID Gene (5′-3′) NO: (5′-3′) NO: P53 CCCAAGCAATGGATG 25GGCATTCTGGGAGCTTCA 36 ATTTGA TCT P21 GGCAGACCAGCATGA 26GCGGATTAGGGCTTCCTC 37 CAGATT TT Bax TGAGCAGATCATGAA 27 GCTCGATCCTGGATGAA38 GACAGGG ACC Noxa AGAGCTGGAAGTCGA 28 GCACCTTCACATTCCTCT 39 GTGT C PumaTCAACGCACAGTACG 29 GTAAGGGCAGGAGTCCC 40 AGCG ATG MDM2 CAGGCAAATGTGCAA 30GGTTACAGCACCATCAGT 41 TACCAA AGGTACAG 14-3-3 GCCTATAAGAACGTG 31CCTCGTTGCTTTTCTGCT 42 GTGGGC CAA CD95 CCCTCCTACCTCTGGT 32TTGAATGTCAGTCACTTG 43 TCTTACG GGCAT Btg2 CCAGGAGGCACTCAC 33GCCCTTGGACGGCTTTTC 44 AGAGC GADD4 CTCAACGTCGACCCC 34 ACATCTCTGTCGTCGTCC45 5 GATAA TCG Survivin CCACCGCATCTCTAC 35 CAAGTCTGGCTCGTTCTC 46 ATTCAAGT

Statistics—Data from the in vitro studies are presented as mean±SD(standard deviation) of sets of data as determined in triplicates.Statistical significance between treatments was determined by Studentt-test, P values <0.05 were considered significant.

In the in vivo studies, the tumor-bearing mice were randomized intovarious treatment groups (n=6) and the tumor volumes were periodicallymonitored and calculated as 4/3π·a·b². Statistical significantdifferences between groups and at different time points were determinedby Student t-test.

Study approval—The study was approved by the Institutional committee foranimal welfare at Tel-Aviv Sourasky Medical Center.

Example 1

Experimental Results

The activity of the PY4 Ras-responsive element was tested in HCT116cells—The activity of the KRAS pathway was evaluated in mutated CRCcells (HCT116). HCT116 cells were transfected with the luciferase vector(and Renilla plasmid) in which luciferase expression is under thecontrol of the SV40 promoter and the PY4 enhancer (the luciferaseconstruct is depicted in FIG. 9B). HCT116 CRC cell line is highlyresponsive to the Ras-activated promoter containing the Py2 Repeats(PY4). The Luciferase activity was normalized to Renilla Luciferaseactivity from a parallel co-transfection (FIG. 1I). As shown in FIG. 1I,the luciferase activity was significantly higher in the transfectedcells compared to the untransfected cells.

Eradication of mutated RAS-harboring cells by adenovirus-mediateddelivery of MazF ribonuclease—The potency and ability of MazF to killthe target cells were evaluated prior to engineering a more complexsystem with several toxicity control points. Massive cell death, in adose-dependent manner, was induced following infection of HCT116 cells[containing a mutated KRAS at codon 13 (Gly to Asp)] withAd-Py4-SV40-mCherry-MazF (FIG. 1A). FIGS. 1A-H show the cytotoxicityinduced by the ribonuclease activity qualitatively evaluated by afluorescent microscope examination 72 hours after the infection (FIGS.1D-E) as compared to the uninfected cells (FIGS. 1B-C). About 35% cellsurvival (relative to the uninfected controls) was quantitativelymeasured by the enzymatic MTT assay upon treatment, when employing a MOIof 25 (FIG. 1F). Cytotoxic activity of MazF was confirmed by FACSanalysis: 50% apoptosis was measured using annexin V (FIG. 1G), whileabout 80% membrane compromised or dead cells was detected with RedDot2(FIG. 1H).

Example 2 Design of a Toxin-Antitoxin Cassese Utilizing Hyperactive Rasin Cancer Cells

Rational design of an innovative toxin-antitoxin cassette for enhancedregulation—The rationale behind the design of the“pAdEasy-Py4-SV40Mp-mCherry-MazF-IRES-TetR-CMVmp-MazE-IRES-EGFP”construct (FIG. 2A), or briefly “pAdEasy-Py4-TA”, was to couple theribonuclease activity with its antidote in order to enable protection ofnon-target cells (i.e., normal cells without a hyperactive RAS pathway)while allowing a high level of expression of the toxic agent in mutatedRAS-harboring cells.

In hyperactive RAS cells (FIG. 2C), the Py4 enhancer element inducedtoxin expression significantly more than that of the antitoxin. The Tetrepressor, which is also expressed in high levels, binds to the Tetoperator sequence and further inhibits the transcription of theantitoxin. Altogether, MazF is expected to overcome the antitoxininhibition and the cells should die.

In cells that do not harbor mutated RAS (FIG. 2D), the Py4 enhancer isnot activated, therefore there is no preference for expression from theSV40 mP. Since the CMV mP is slightly stronger than the SV40 mP and onemolecule of the AT inhibits two molecules of toxin, the inhibitoryactivity of the antitoxin should prevail.

Consequently, the MazE in these cells will overcome the toxicity of MazFand the cells will survive. FIG. 2E shows a representative Western blotthat confirms the differences in the degree of expression of the toxin(represented by the mCherry) vs. antitoxin (represented by the GFP). Incells with mutated KRAS, the expression of the toxin is higher than theantitoxin upon infection with the PY4-TA viruses.

Example 3 Maze Protects Normal Cells from MAZF Cytotoxic Activity MazEProtects Normal Cells from MazF Cytotoxic Activity

Experimental Results

In order to demonstrate the advantage of using the pAdEasy-Py4-TAcassette, the present inventors tested its ability to protect cells withwild type (WT) RAS from possible “leakage” of the lethal gene. The basalexpression from the SV40 mP along with low expression levels of RAS innormal cells induce low expression of MazF. However, even this low levelof expression is sufficient to kill a cell. HT29 cells, with WT RAS,were infected with twofold dilutions of the MazEF- or MazF-encodingviruses, and the viability of the cells was qualitatively examined bylight and fluorescence microscopy. As shown in FIGS. 3C, 3E, 3G, 3I, and3K, infection with MazF decreased cell viability, indicating a leakinessof MazF expression even in the absence of mutated RAS. In contrast,infection with the MazEF construct was well tolerated. When visualizedunder a fluorescence microscope, the intoxicatedAd-Py4-SV40-mCherry-MazF-infected cells showed very faint redfluorescence, indicating inefficient mCherry-MazF accumulation. This isdue to the ribonuclease activity of MazF that results in inhibition ofprotein synthesis, including its own (Zhang 2003; Shapira et al., 2012).On the other hand, the ribonuclease activity of MazF was neutralized byits antidote MazE in cells infected with pAdEasy-Py4-TA, as indicated bythe presence of both red and green fluorescence (FIGS. 3B, 3D, 3F, 3Hand 3J).

Example 4 The MAZEF Cassette is Capable of Killing Cells HarboringHyperactive RAS

Experimental Results

Adenovirus-mediated delivery of TA-encoding cassette specificallyeliminates RI cells harboring activated RAS—The potential of theMazEF-encoding cassette to kill target cells was tested in a proof ofconcept study performed in R1 cells, which serve as a model system forhyperactive RAS-harboring cells (Arber et al., 1996). R1 cells wereinfected with twofold dilutions of the mCherry encoding virus (themCherry construct is schematically depicted in FIG. 4A) or of the PY4-TA(MazEF-encoding) virus (FIG. 2A). Infection of R1 cells with MazEFelicited a considerable cytotoxic effect, decreasing viability to 36%(at MOI 5) relative to the uninfected controls, while no significanteffect was seen after infection with the mCherry cassette (80-90%)(shown qualitatively by light microscope in FIGS. 4B-D andquantitatively by MTT in FIG. 4E). The expression of the GFP and mCherryproteins indicates that both MazF and MazE components had been expressed(FIGS. 4F-G). However, although both, mazF and mazE were expressed, themazF overcame the inhibition of the antitoxin in KRAS mutated cells;

The MazEF cassette kills mutated RAS-harboring cells—An in vitrocolony-forming assay was performed to qualitatively and comparativelyassess the sensitivity of CRC cells with mutated RAS to the expressionof the transgene. In addition, this assay was intended to verify thatMazF is well tolerated by the normal cells that do not harborhyperactive RAS. HCT116 (harboring hyperactive RAS) and HT29 (no KRASmutation) cells were infected with 25 and 10 MOI of “pAdEasy-Py4-TA”,“pAdEasy-ΔPy4-TA” or left uninfected. The cells were trypsinized andseeded in 3-fold dilutions 7 hours later. Surviving colonies werestained after 7 days. FIGS. 5A-J show the potency of the expressedtransgene under the control of the RRE (FIG. 5B) compared to uninfectedcells (FIG. 5A) and to the control cassette carrying a deletion of theRRE (FIG. 5D). Prominent differences in the numbers of survivingcolonies were observed, confirming that MazF indeed overcame theinhibitory effect of MazE in mutated RAS-harboring cells (FIG. 5B) whileMazE was able to protect cells (with the wild type RAS, devoid of the RAhyperactive mutation) from MazF toxicity (FIG. 5G). In addition, theselectivity of this targeting system was confirmed since the massivecell death took place only in the RRE—including cassette (FIG. 5B),while no significant effect was seen after infection with the ΔPy4-TAcassette that lacks the RRE (FIG. 5D).

Deletion of the RAS-responsive element decreases the cytotoxic activityof MazF—As mentioned above, an additional cassette lacking the RRE,“pAdEasy-ΔPy4-TA”, was also constructed (FIG. 2B). FIGS. 7A-Cdemonstrate that deletion of the RAS-responsive DNA element enhanced thecontribution of MazE. Cell viability was measured by FACS analysis:while massive cell death (55% apoptosis, 82% dead cells) was observedfollowing infection with the full toxin-antitoxin encoding viruses,deletion of the RAS-responsive DNA element dramatically reduced thiseffect to 18% and 10%, respectively (FIGS. 7A-B; the untreated cells areshown in red, the toxin antitoxin-treated cells are shown in black, andthe cells that were treated with the RRE deletion cassette are shown ingreen). These results were also confirmed by the enzymatic MTT assay(FIG. 7C), where a difference of about 60% was observed between Py4-TAand ΔPy4-TA.

Adenovirus-mediated delivery of TA encoding cassette specificallyeliminates activated RAS-harboring CRC cells—The present inventorsfurther examined the ability of the above-described TA system to kill ahuman cancer cell line expressing hyperactive RAS. HCT116 and HT29 cellswere infected with twofold dilutions of the MazEF-encoding viruses,starting from 10 MOI (FIGS. 6E-J), or left untreated (FIGS. 6A-D).Massive cell death (73%, relative to the uninfected control, at 10 MOI)was demonstrated exclusively in the mutated KRAS harboring cells(HCT116) but not in cells lacking hyperactive RAS (HT29 cells),emphasizing the potency of this system (FIG. 6K). The percentage andintensity of the fluorescence of green cells (FIG. 6J) was higher thanthat of the red ones (FIG. 61) in HT29 cells, lacking hyperactive RAS.This indicates that the expression of the antitoxin increases andexceeds that of the toxin in cells devoid of the hyperactive RAS.

In order to confirm and further support the ability of this system toprotect normal cells on one hand, and to efficiently kill cancer cellson the other hand, additional CRC cell lines were tested and yieldedvery similar results, substantiating the above observations (data notshown). The potency of this system was also tested for other cancertypes, such as pancreatic and prostate, suggesting a wide range oftherapeutic potential of this suggested treatment modality (data notshown).

Induction by tetracycline results increased expression of the antitoxinMazE in naïve cells—Inclusion of tetracycline provided an additionalprotective layer, not only for virus production and propagation but alsofor normal cell protection. Binding of tetracycline to the Tet repressorled to a conformation change that resulted in relief of the MazEtranscriptional inhibition. Consequently, expression of the antitoxinwas increased in naïve cells, as demonstrated by the increase in greenfluorescence (FIGS. 6N and 50), while tetracycline did not compromisethe toxicity of MazF in mutated RAS-harboring cells (FIG. 6P). It isnoted that in cells with mutated RAS with or without the addition oftetracycline, the viability of the cells was the same.

Example 5 MAZEF-Encoding Viruses Inhibit Tumor Growth In Vivo

Experimental Results

MazEF-encoding viruses inhibit tumor growth in vivo—The therapeuticpotential of the TA system was tested by specific targeting of tumorcells in nude mice bearing a xenograft of HCT116 CRC cells, harboringmutated RAS. The growth of these cells was markedly inhibited byPy4-TA-encoding viruses (FIG. 8A). Impressive tumor shrinkage wasdemonstrated in vivo following treatment with Ad-Py4-SV40-MazEF-encodingadenovirus (61%) (P<0.0002) without any toxic or side effects. In theAd-ΔPy4-SV40-MazEF treated mice (control group) tumor volume was reducedby only 27% (P<0.4). No growth inhibition was seen following injectionof PBS (FIG. 8A).

Throughout this study the mice were monitored for their generalwell-being, weight, food and water consumption. At no time there was anyevidence of toxicity ascribable to the adenoviral vectors (data notshown).

The expression of MazF and MazE in the infected tumor cells wasmonitored with the Maestro imaging CRi device (FIG. 8B). The imaging wasperformed on live mice (FIG. 8B) and outside the mouse body (FIG. 8C).This analysis confirmed that the adenoviruses indeed targeted themutated RAS-harboring tumors and that the MazEF genes were expressed.

Example 6 An Orthotopic Model for Testing the Constructs

The present inventors have set up an orthotopic model for testing theefficacy of the therapeutic constructs system. For that purpose, the R1cells (enterocytes that constitutively express mutated KRAS) areinitially used. Additionally or alternatively, a similar system inmurine cells (mc38 cell line) which express the KRAS mutated oncogene iscurrently designed.

Study design: The cells are injected into the colon of the mice, theinitial volume of the tumors is measured by colonoscopy and then thetreatment by a systemic injection of the viruses begins.

Since the orthotopic model is performed in C57/bl mice, the presentinventors first tested whether injection of the R1 cell line can grow toform tumors in the mouse. For that purpose, 5×10⁶ R1 cells weresubcutaneously injected at two sites on the backs of the mice. All themice (5/5) developed tumors (data not shown). The viruses used for thisexperiment were propagated, produced and their titer was determined.

Example 7 Establishment of Stable Transfected System for KRAS Mutation

The present inventors have designed and constructed an additional vectorwhich encodes for the mutated kras having a missense mutation G13D inthe KRAS (FIG. 9A). FIG. 9B schematically illustrates the luciferaseconstruct. Next, a stable transfection to the mc38 murine coloncarcinoma cells was performed and by measuring the luciferase expressionthe best clone was screened and selected (FIG. 9B), and as shown, thebest clone was C3.

Example 8 AAV (Adeno Associated Virus)

The limitations of using adenovirus are well known, among other reasonsbecause it causes an immune response. Therefore, the present inventorscurrently establish less immunogenic delivery systems; such as the AAVdelivery system.

The AAV is one of the smallest ‘viruses, it is a single-stranded DNA.Its properties made it one of the most promising delivery systems,partly because of its low immunogenicity, the long duration oftransgenic expression, and because there are many serotypes to AAV andeach one of them can infect only specific cell type, therefore theselectivity of the virus is high.

Identification of the most appropriate serotype for the different targetcells, especially colorectal and pancreatic cancer cells—For thatpurpose, the present inventors induced tumors that derived fromdifferent cell lines in nude mice. Then a systemic single infecting ofthe various serotypes in several titers were conducted, and after twoweeks the mice were sacrificed, the tumors were removed and theexpression of the GFP was evaluated by Western blot analysis as shown inFIG. 10. According to these results, the AAV serotype 6 is the bestserotype for the HT29 target cells.

The PY4-mazEF and ΔPY4-mazEF cassettes were cloned to AAV serotype 6 andthe viruses are produced. These particles are evaluated in vivo, in nudemice bearing xenograft of pancreatic cancer cells.

Example 9 Establishment of a Dual System Based on RAS and P53 ResponsiveElements

For establishing the dual system adenoviral vectors carrying the toxin(PY4-MazF-mcherry) and the antitoxin (RGC-MazE-IRES-GFP) were clonedunder the regulation of Ras responsive elements and p53 responsiveelements, respectively. FIG. 11A provides a schematic illustration ofsuch a dual system.

Virus particles were produced, their titer was calculated and theirpotency was tested in vitro. Cell death was measured qualitatively byusing the fluorescent microscopy and was quantified by the enzymatic MTTassay.

A594, Ras^(mut)/p53^(wt); H2030, Ras^(mut)/p53^(wt); H1299,Ras^(mut)/p53^(mut); H1650, Ras^(wt)/p53^(wt) and H1975,Ras^(wt)/p53^(mut)—lung cancer (LC) cell lines were used as a modelsystem for testing the potency of the adenoviruses-based system. MiaPaca, Ras^(mut)/p53^(mut); Colo357, Ras^(mut)/p53^(mut) Panc1,Ras^(mut)/p53^(mut) and BxPC3, Ras^(wt)/p53^(mut)-pancreatic cancer celllines were tested as well.

Co-infection assays were performed, using the optimal 1:0.5 MOI ratio.The results showed decrease in the mortality of the mutated Ras cellsexpressing wild type (WT) p53; 36% with a titer of 7.5 MOI (FIG. 12).These results indicate that cells, which have WT p53, that expressed thetoxin were protected by the antitoxin expressed under the p53 responsiveelement, while cells that have mutations of both genes, i.e., ras andp53, such as the SHP77, showed increased sensitivity.

The efficacy of the above dual system is tested in vitro and in vivo.

Example 10

The present inventors further tested whether the RAS responsive PY4element works as an enhancer only in cells that carry mutation of theKRAS or even of N- and H-RAS genes. For that purpose, the ability of allthe three RAS oncogenes (HRAS, NRAS, and KRAS) to activate theirpathways was examined by testing their ability to stimulate theirdownstream transcription factors. H1299 (NRAS oncogene expressing cellline), A549 (KRAS oncogene expressing cell line) and T24 (HRAS oncogeneexpressing cell line) were co-transfected with PY4-luciferase andRenilla luciferase plasmids. The results presented in FIG. 13A showedthat the all three are able to induce the transcription of theluciferase reporter gene, with different efficiency (transcriptionlevels of PY4-luciferase were normalized to the Renilla luciferaseactivity).

Next, the present inventors examined the ability of those threeoncogenes to induce the transcription of the toxin under the regulationof PY4 element. H1299, H2030, A549, and T24 were infected withAd-PY4-mcherry-mazF viruses. Cell viability was measured by the MTTassay (72 hours post infection). These experiments support the previousobservations by showing the different levels of toxin expression whichin turn are leading to different percentage of cell viability (FIG.13B).

Example 11 Chemically Induction of P53 Expression in CRC Cells P53Responsive Element

Experimental Results

In order to test the ability of p53 to bind to its responsive elementand stimulate transcription, HCT116 cells were transfected withRGC-mazE-IRES-GFP plasmid. Since p53 is degraded in un-activated cells,the present inventors used Quercetin, a ubiquitous bioactive plantflavonoid that is able to induce p53 phosphorylation, stabilization andtotal p53 protein accumulation. Another widely used p53 inducer is theantimetabolite agent 5-FU. P53 transcription activity was evaluated bymeasuring the expression of the downstream reporter gene, GFP. Thepresent inventors show that the addition of Quercetin (50 μM) to thetransfected HCT116 cells increased the expression of GFP by about 10folds. Addition of 5-FU has led to significantly higher expressionlevels of GFP, even in lower concentration (10 μM). Furthermore,Quercetin showed severe cell toxicity (60% cell death) while no toxicityhas been shown upon treatment with 5-FU, even very high doses (data notshown).

Induction of the transcription of P53 target genes—Further validation ofendogenous p53 activation led the present inventors to test othercanonical target genes. 5-FU (50 μM) was added to HCT116 p53 WT cellsfor 24 hours. Total cell lysate was prepared and subjected into Westernblot analysis and at the same time RNA was prepared and used as atemplate for cDNA and semi-quantitative PCR was performed. The resultsshow and confirm that protein expression, of all the target genes,correlates to the mRNA up regulation due to p53 activation. Inparticular a tight correlation between p21 and p53 protein levels (FIGS.14A-C).

An in vitro colony-forming assay was performed to qualitatively andcomparatively assess the sensitivity of lung cancer cells with mutatedRAS to the expression of the transgene. In addition, this assay wasintended to verify that MazF is well tolerated by the normal cells thatdo not harbor hyperactive RAS. A549 and H1650 cells were infected with10 MOI of the adeno-PY4-mazF-mcherry and adeno-RGC-mazE-GFP viruses, ina ratio of 1:0.5 ratio, respectively. In parallel, those cells wereinfected with adeno-ΔPY4-mazF-mcherry and adeno-RGC-mazE-GFP viruses, ina ratio of 1:0.5, respectively. Empty vector (CMV-mcherry) infected oruninfected cells used as a control. After 7 hours, the cells weretrypsinized and seeded in 3-fold dilutions and incubated for 7 days.Surviving colonies were fixed with 4% formaldehyde in PBS and stainedwith 0.02% crystal violet. A549 KRAS mutated and p53 WT expressing cellsthat were infected with PY4-mazF-mcherry and RGC-mazE-GFP viruses showedlower survival rate compared to ΔPY4-mazF-mcherry and RGC-mazE-GFPviruses infected cells. However, H1650 KRAS WT and p53 WT expressingcells that were infected with PY4-mazF-mcherry and RGC-mazE-GFP virusesshowed no significant fold change in survival compared toΔPY4-mazF-mcherry and RGC-mazE-GFP viruses infected cells (FIGS. 16A-H).

A549 cells express the mutated KRAS oncogene therefore the expression ofthe mazF toxin is high. In addition, these cells carry WT form of p53that binds to its responsive element and enhances the transcription ofthe mazE anti-toxin. On the contrarily, H1650 cells express WT RAS,therefore the transcription of the toxin wasn't enhanced. Consequently,there was not a significant difference in toxicity between cells thatwere infected with viruses that carry or not the RAS responsiveelements. The presence of WT p53 in these cells leads to the expressionof the anti-toxin. Both the toxin and the antitoxin were visualized bythe expression of the fluorescence proteins (mcherry and GFP,respectively). It is important to note the two cell lines above aredifferent in their sensitivity to viral infections. H1650 cells showedmuch lower survival level then A549 line, upon infection with an emptyvector (FIGS. 16A-H).

The efficacy of mazF to kill has been evaluated also in pancreaticcancer cells—PANC1, Mia Paca2, Colo357 (KRAS mutated cells) and BxPC3(wt RAS) cell lines were seeded in 96-well plates. After 24 hoursdifferent dilutions of PY4-mazF-mcherry viruses were added. 72 hourslater, cell survival was measured the enzymatic MTT assay. The resultsshow that cells with hyperactive KRAS were more sensitive, about 50%viability in MOI of 15. However, WT RAS expressing cells showed 80%viability under the same conditions (FIG. 17).

Analysis and Discussion

In the present study vectors for cancer-directed gene delivery wereconstructed; “pAdEasy-Py4-SV40mP-mCherry-MazF”,“pAdEasy-Py4-SV40mP-mCherry-MazF-IRES-TetR-CMVmp-MazE-IRES-EGFP”,“pAdEasy-ΔPy4-SV40mP-mCherry-MazF-IRES-TetR-CMVmp-MazE-IRES-EGFP” and“pAdEasy-mCherry”. Virus particles were produced and their potency wastested. Cell death was measured qualitatively by using the fluorescentmicroscopy and colony formation assay, and was quantified by MTT. FACSanalysis using annexin V and RedDot2 dyes was performed for measuringapoptosis and dead cells, respectively. In vivo tumor formation wasmeasured in xenograft model.

Herein, an improved approach is suggested by tightening the expressionof the toxin and replacing the pro-apoptotic gene by a significant morepotent toxic molecule that does not exist in human cells.

These results demonstrate a very well-regulated system that canprecisely control gene delivery and expression at a specific targetedsite. This system exploits the hyperactive RAS pathway, rather thaninhibiting it. In addition, the results presented here demonstrate aproof-of-concept that normal tissues can be selectively spared from thetoxic effects of drugs by taking advantage of their wt RAS that expressthe “antidote”.

Thus, the MazF-MazE toxin-antitoxin system has a potential to be used asa therapeutic tool, to kill undetectable micrometastases that are amajor hurdle in challenging.

This approach, of taking advantage of Ras mutation in order toselectively kill cancer cells while sparing the normal cells, eitheralone or preferably in conjunction with other treatment modalities canenhance efficacy while reducing toxicity.

Thus, the suggested gene therapy strategy can advance the management ofhuman cancer, allowing a tailor-made protocol for biological treatmentspecific to the molecular profile of a tumor.

Although the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims.

All publications, patents and patent applications mentioned in thisspecification are herein incorporated in their entirety by referenceinto the specification, to the same extent as if each individualpublication, patent or patent application was specifically andindividually indicated to be incorporated herein by reference. Inaddition, citation or identification of any reference in thisapplication shall not be construed as an admission that such referenceis available as prior art to the present invention. To the extent thatsection headings are used, they should not be construed as necessarilylimiting.

Although the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims.

All publications, patents and patent applications mentioned in thisspecification are herein incorporated in their entirety by referenceinto the specification, to the same extent as if each individualpublication, patent or patent application was specifically andindividually indicated to be incorporated herein by reference. Inaddition, citation or identification of any reference in thisapplication shall not be construed as an admission that such referenceis available as prior art to the present invention. To the extent thatsection headings are used, they should not be construed as necessarilylimiting.

REFERENCES Additional References are Cited in Text

-   1. American Cancer Society. Colorectal Cancer Facts & Figures. 2014.-   2. Reddy M A, Langer S J, Colman M S, et al. An enhancer element    responsive to ras and fms signaling pathways is composed of two    distinct nuclear factor binding sites. Mol Endocrinol 1992;    6:1051-60.-   3. Nabel G J. Genetic, cellular and immune approaches to disease    therapy: past and future. Nat Med 2004; 10:135-41.-   4. Kootstra N A, Verma I M. Gene therapy with viral vectors. Annu    Rev Pharmacol Toxicol 2003; 43:413-39.-   5. Verma I M, Weitzman M D. Gene therapy: twenty-first century    medicine. Annu Rev Biochem 2005; 74:711-38.-   6. Young L S, Searle P F, Onion D, et al. Viral gene therapy    strategies: from basic science to clinical application. J Pathol    2006; 208:299-318.-   7. Kaplan J M. Adenovirus-based cancer gene therapy. Curr Gene Ther    2005; 5:595-605.-   8. El-Aneed A. Current strategies in cancer gene therapy. Eur J    Pharmacol 2004; 498:1-8.-   9. Dvory-Sobol H, Kazanov D, Arber N. Gene targeting approach to    selectively kill colon cancer cells, with hyperactive K-Ras pathway.    Biomed Pharmacother 2005; 59 Suppl 2:S370-4.-   10. Dvory-Sobol H, Sagiv E, Kazanov D, et al. Targeting the active    beta-catenin pathway to treat cancer cells. Mol Cancer Ther 2006;    5:2861-71.-   11. Dvory-Sobol H, Sagiv E, Liberman E, et al. Suppression of    gastric cancer cell growth by targeting the beta-catenin/T-cell    factor pathway. Cancer 2007; 109:188-97.-   12. Giladi N, Dvory-Sobol H, Sagiv E, et al. Gene therapy approach    in prostate cancer cells using an active Wnt signal. Biomed    Pharmacother 2007; 61:527-30.-   13. Naumov I, Kazanov D, Lisiansky V, et al. Novel approach to abuse    the hyperactive K-Ras pathway for adenoviral gene therapy of    colorectal cancer.

Exp Cell Res 2012; 318:160-8.

-   14. FitzGerald D, Pastan I. Redirecting Pseudomonas exotoxin. Semin    Cell Biol 1991; 2:31-7.-   15. Lisiansky V, Naumov I, Shapira S, et al. Gene therapy of    pancreatic cancer targeting the K-Ras oncogene. Cancer Gene Ther    2012; 19:862-9.-   16. Inouye M. The discovery of mRNA interferases: implication in    bacterial physiology and application to biotechnology. J Cell    Physiol 2006; 209:670-6.-   17. Pandey D P, Gerdes K. Toxin-antitoxin loci are highly abundant    in free-living but lost from host-associated prokaryotes. Nucleic    Acids Res 2005; 33:966-76.-   18. Engelberg-Kulka H, Amitai S, Kolodkin-Gal I, et al. Bacterial    programmed cell death and multicellular behavior in bacteria. PLoS    Genet 2006; 2:e135.-   19. Engelberg-Kulka H, Hazan R, Amitai S. mazEF: a chromosomal    toxin-antitoxin module that triggers programmed cell death in    bacteria. J Cell Sci 2005; 118:4327-32.-   20. Cook G M, Robson J R, Frampton R A, et al. Ribonucleases in    bacterial toxin-antitoxin systems. Biochim Biophys Acta 2013;    1829:523-31.-   21. Thisted T, Sorensen N S, Wagner E G, et al. Mechanism of    post-segregational killing: Sok antisense RNA interacts with Hok    mRNA via its 5′-end single-stranded leader and competes with the    3′-end of Hok mRNA for binding to the mok translational initiation    region. EMBO J 1994; 13:1960-8.-   22. Gerdes K, Bech F W, Jorgensen S T, et al. Mechanism of    postsegregational killing by the hok gene product of the parB system    of plasmid R1 and its homology with the relF gene product of the E.    coli relB operon. EMBO J 1986; 5:2023-9.-   23. Zhang Y, Zhang J, Hoeflich K P, et al. MazF cleaves cellular    mRNAs specifically at ACA to block protein synthesis in Escherichia    coli. Mol Cell 2003; 12:913-23.-   24. Shapira A, Shapira S, Gal-Tanamy M, et al. Removal of hepatitis    C virus-infected cells by a zymogenized bacterial toxin. PLoS One    2012; 7:e32320.-   25. Arber N, Sutter T, Miyake M, et al. Increased expression of    cyclin D1 and the Rb tumor suppressor gene in c-K-ras transformed    rat enterocytes. Oncogene 1996; 12:1903-8.-   26. Okamoto M, Chono H, Kawano Y, et al. Sustained inhibition of    HIV-1 replication by conditional expression of the E. coli-derived    endoribonuclease MazF in CD4+ T cells. Hum Gene Ther Methods 2013;    24:94-103.-   27. Chono H, Matsumoto K, Tsuda H, et al. Acquisition of HIV-1    resistance in T lymphocytes using an ACA-specific E. coli mRNA    interferase. Hum Gene Ther 2011; 22:35-43.-   28. Shimazu T, Mirochnitchenko O, Phadtare S, et al. Regression of    Solid Tumors by Induction of MazF, a Bacterial mRNA    Endoribonuclease. J Mol Microbiol Biotechnol 2014; 24:228-33.

1. A nucleic acid construct system comprising: (i) a first nucleic acidconstruct encoding a toxin operatively linked to a first promoter and atleast one cancer-associated signaling responsive enhancer element; (ii)a second nucleic acid construct encoding an anti-toxin operativelylinked to a second promoter, said second promoter being stronger thansaid first promoter.
 2. The nucleic acid construct system of claim 1,wherein said first nucleic acid construct and said second nucleic acidconstruct are provided at substantially equal concentrations.
 3. Thenucleic acid construct system of claim 1, wherein said toxin andantitoxin are a bacterial-derived toxin-antitoxin pair.
 4. The nucleicacid construct system of claim 3, wherein said toxin-antitoxin pair is abacterial type II toxin-antitoxin pair.
 5. The nucleic acid constructsystem of claim 4, wherein said toxin-antitoxin pair is selected from:CcdB-CcdA, ParE-ParD, MazF-MazE, yafO-yafN, HicA-HicB, Kid-Kis, andZeta-Epsilon.
 6. The nucleic acid construct system of claim 1, whereinsaid first promoter and said second promoter are both constitutivepromoters.
 7. The nucleic acid construct system of claim 1, wherein saidat least one cancer-associated signaling responsive enhancer element isa Ras responsive enhancer element.
 8. The nucleic acid construct systemof claim 7, wherein said Ras responsive enhancer element is a K-Rasresponsive enhancer element.
 9. The nucleic acid construct system ofclaim 7, wherein said Ras responsive enhancer element comprises at leastone element selected from the group consisting of an Ets binding site,an Ap-1 binding site and a PY2 sequence.
 10. The nucleic acid constructsystem of claim 1, wherein said antitoxin is operatively linked to saidsecond promoter and a p53 wild-type responsive element.
 11. The nucleicacid construct system of claim 10, wherein said p53 wild-type responsiveelement comprise 17 copies of a p53 wild type responsive element as setforth in SEQ ID NO:
 15. 12. The nucleic acid construct system of claim1, wherein said second promoter comprises CMV and said first promotercomprises SV40.
 13. The nucleic acid construct system of claim 1,wherein said first nucleic acid construct, said second nucleic acidconstruct or both is adeno-virus based or is lenti-virus based.
 14. Thenucleic acid construct system of claim 1, wherein said first nucleicacid construct further comprises a repressor of a bacterialrepressor-operator system, said repressor being under a transcriptionalregulation of said cancer-associated signaling responsive enhancerelement, and wherein said second nucleic acid construct comprises anoperator of said bacterial repressor-operator system, such thatexpression of said repressor inhibits expression of said antitoxin. 15.The nucleic acid construct system of claim 14, a. wherein said repressorcomprises the Tetracycline repressor (Tet-R) sequence, and wherein saidoperator comprises the tetracycline operator sequence; b. wherein saidoperator comprises at least two repeats of the sequence tetracyclineoperator sequence; or c. both.
 16. The nucleic acid construct system ofclaim 1, wherein said first nucleic acid construct comprises fourrepeats of the PY2 sequence set forth by SEQ ID NO:2 being upstream andoperably linked to the SV40 minimal promoter region set forth by SEQ IDNO:4, a toxin coding sequence being downstream of and transcriptionallyregulated by said SV40 minimal promoter region, an IRES sequence setforth by SEQ ID NO:7 being downstream and operably linked to said toxincoding sequence, and a Tetracycline repressor set forth by SEQ ID NO: 8being downstream of and operably linked to said IRES sequence.
 17. Thenucleic acid construct system of claim 1, wherein said second nucleicacid construct comprises a CMV minimal promoter which comprises tworepeats of a tetracycline operator as set forth by SEQ ID NO:9 and anantitoxin coding sequence being downstream of and operably linked tosaid CMV minimal promoter.
 18. A pharmaceutical composition comprisingthe nucleic acid construct system of claim 1 and a pharmaceuticallyacceptable vehicle.
 19. A method of treating cancer in a patient in needthereof, said method comprising administering to said subject aneffective amount of the pharmaceutical composition of claim 18, therebytreating said cancer patient.
 20. The method of claim 19, wherein saidcancer comprises a tumor overexpressing Ras and said tumor cells arecharacterized by a hyperactive RAS pathway as compared to non-tumorcells of the same tissue.